I think there are three major problems with difficulty scaling. I'll discuss them first and then propose possible solutions near the end.
1) the number scale
2) lack of lesser monsters
3) encounter structure
1) The number scale. There are two aspect to scaling numbers, the first is numbers relative from one level to the next, and the second is the range of numbers from the minimum reasonably expected number to the maximum reasonably expected number.
Now, in the ttrpg, there was already a problem with the range from minimum to maximum values growing too large at high levels. I suspect this is even worse in DDO especially given all the new ways to add more modifiers, which makes the range from someone who has put moderate resources into a value vs those who have skyrocketed those values is large. You have enhancement bonuses to weapons and abilities which add more bonuses, then you have insight and exceptional bonuses, defense mitigation, and a slew of others. The range between casual and hardcore and "I-have-no-life-outside-the-game" players is so drastic that making content to challenge one is either too easy or impossible for the others, just numerically.
Even something like invisibility becomes meaningless very quickly.
Of course, in other cases the problem is reversed. There is a quest in the harbor in which one has to get past kobolds without killing the priests. I tried using a sleep spell. Even though I was so far above them in level that I didn't get xp for the quest, my sleep spell affected like one kobold in the entire quest. I had great int bonus and everything, yet it basically made the tactic, and the spell, pointless.
There are several cases where moderate ability in something becomes pointless because the stats on enemies are so high to deal with players that stack as many bonuses as possible, and thus leave anyone else falling back on pure dmg to overcome enemies. Heck, I win with battlefield tactics which still boil down to applying dmg because no cc effects ever work.
For the longest time I thought this was just because I was always facing opponents 4-6 levels above mine, but now that I'm at higher levels where the difference between lvls is so vast that fighting 1-2 levels above my level has become very difficult, I still don't land anything. Then I went around doing some odd low level quests I hadn't done before mostly for story, only to find that even being massively overleveled wasn't enough to do much other than win through DPS.
2) No lesser monsters. Frankly, the lower level spells become nearly useless at high levels. Largely because there is no one to use them on. The lesser encounters are often at least on par with player level so none are low enough to fall for lesser spell effects, making them pointless.
3) Encounter structures. Encounters seem to be built around dps. There is little organization, and little variety in tactics. The kobold assault quest in the harbor is the only one I know of that tries to overwhelm with numbers, except even that is hindered by breaking the numbers up into much smaller groups losing any advantage that greater numbers provide. There are no formations, little in the way of enemies providing support, and enemies are always stupid. I mean I know that making AI smarter is difficult, though the AI can easily be improved even if running on decision trees and aggro, but aside from that, the design of encounter groups is not very good. The use of terrain is not good.
4) The lack of feeling powerful. In the ttrpg, lvl 5 was the most powerful any real world person could ever expect to be, and lvl 15-20 is like Superman from the comicbooks. While DDO might not follow the exact same numerical expectations, there is a problem when being a lvl 20 feels very much the same as level 5. Always facing enemies of about equal power and the impact on the environment is always the same, so I never feel like a powerful character because big numbers alone will never make me feel powerful, I need to see it in the results of those numbers. Taking out a monster in one hit that used to be a boss that I couldn't solo would be a good example, except such a thing almost never happens.
Fixes.
First, the number squash that happened recently, I think the idea was good, but too limited. Why was it only legendary? I think the whole graph of number ranges needs to be shrunk and smoothed. Have a measure of the different types of bonuses that can apply to any one thing and set absolute limits for the different categories and make a smooth and slow transition from level 1 to level 30, such that A) the difference from level 2 to 3 is nearly the same as from 27 to 28, and B) such that the minimum that can be reasonably expected and the maximum that can be reasonably expected is around a difference of 20, or at most 30, and C) that going beyond that reasonable maximum, if possible at all, requires hefty drawbacks to other equally important stats, like giving a +2 attack beyond that max requires a -4 or even -6 to AC. Making it something that really only works in group with people using tight-knit teamwork.
Spell DCs range from X to X+9, so this leaves room for up to +10 DC from other sources. Of course, the ttrpg range of ability scores, from 3-20 gives up to +5, of course adding so much to ability scores raises that a lot.
These numbers also need to be linearly different rather than proportional, since proportionally different values will only diverge more and more.
So for example, high, medium, and low bab can be lvl, lvl -2, and lvl-4 respectively, making the difference the same from level 1 to 30. Then at certain points we can expect new types of bonuses to make a difference to counteract the aspect of a difference of 4 meaning less relative to the total attack bonus at higher levels.
Thus, you might say that when it comes to numbers, you have the core stat (bab, skill ranks, etc) which scales by level, then ability score bonuses ranging from -2 to +10, enhancement and insight each range from +1 to +6, and all other bonuses cap out at +4 and never stack more than two of them for +8 from these other bonuses. This means that at max lvl with every last bonus achievable, there is only +30 over the base stat. From this core, special bonuses such as from the DDO store items giving up to +2 or even +4 won't be much of a problem and can still be sold, but also makes it much easier to keep the numbers from growing so far out of control that optimization is required to achieve anything.
So then the question comes to advancement, if you only gain up to +30 from all bonuses over the course of a life, that is roughly +1 per lvl, which many might say is rather lackluster. Well, indirect growth can help here. For example, instead of bonuses directly improving ability scores, make it so that any bonuses to ability scores are like additional ability score points like those spent at chargen, thus more and more points are needed for each +1 growth of the ability score. Doing that means that even a player starting with a score of 20 needs an extra +80 points to reach an ability score of 30. That is plenty of space to add all kinds of bonuses to ability scores while making sure that the actual effect of those bonuses on combat are useful yet not overwhelming.
The misc bonuses that only stack a max of two is a chance for making builds a creative endeavor that still seeks plenty of loot. For example, if a good alchemical bonus is found, then maybe that only stacks with divine bonus, leading the player to seek out a good divine bonus so that it will stack with their alchemical bonus. Plenty of loot options and variety of builds without bonkers crazy number growth.
However, the biggest key here is A) making things other than numbers be very important, and B) making the important numbers not simultaneously improved. This means that environmental effects and positioning as important factors needs to be made stronger, as well as battlefield tactics. And AC and attack should be improved separately, not just by requiring separate stats to improve, but by having an item that improves one, not improve the other, nor provide two bonuses to the same thing. Equipping multiple items should be a bit like a puzzle which makes it impossible to add all the bonuses one wants.
As for monsters and combat. I expect these to be decision trees, though obviously I'm not certain. But if that's true, there is a lot that can be done to improve them. But even if not, the encounters can be improved by the design of the encounter groups. Firstly, including more lower level monsters in large groups as lvl goes up increases the feeling of being powerful while still adding challenge. Imagine the kobold assault for epic lvl 20 where the leader is a lvl 20 trog while the chieftain is a lvl 18 with 6-8 lvl 15s elites and the rest of the 200 kobolds are lvl 1-10. Now have them attack 20-40 at a time with multiple units, where you have a squad of shield tanks getting close, and a squad of spearmen behind them followed by a squad of fire throwers with a few healers scattered in there. That would be challenging, especially with a few sorcerers to dispel cloud and AoEs. These kobolds wouldn't be individually challenging at all, but in a massive group with this kind of group design, and they'd be a problem. And this is just an off-the-top of my head build. Spend a little bit of time and this can be made even more difficult without increasing stats at all. And the neat aspect is that players can still feel powerful because they will obviously be far better than the individual kobolds, with them falling after 1-3 hits.
And dmg needs this numerical treatment as well. Especially on spells. Part of the problem here is the idea of raising difficulty simply by increasing HP. Terrible idea. I play SAOIF and solo guild raid bosses by spending 20-30 minutes chipping away at HP. Not challenging. The same applies here, simply raising HP does not make a battle more challenging. Certainly enough HP is needed to not die without taking any actions, but beyond that, simply raising HP is the wrong move. I hate those fights the most, the ones where the enemy has literally tens of thousands of HP against my 400. If they need that much HP just to challenge me, especially someone like me that doesn't do any optimization nor has many bonuses, and my defense rating at lvl is sitting as 37-43%, and those high HP guys still lose, that is a sign of bad design, too much focus on the dmg vs hp aspect and not enough on tactics, cc, type strengths/weaknesses, etc.
Personally, I think the entire reason the numbers got out of control in the first place with the numbers getting bigger every expansion is that the design hasn't been looked at as anything other growing numbers.
What is needed for this is A) increased tactical differentiation and B) more scope for versatility. Make it so a lot of advancement is about gaining versatility rather than numbers, and make that versatility useful.
Well, I;m out of time. Hopefully this sparks some thought and productive discussion.