You can't adjust DCs to make them work well across all of the content/difficulty settings for the following reasons:
1. The range of available player power is far too wide. Imagine if all the DCs were adjusted to the MAX necro DC a player could possibly have. That would mean that players without access to EVERY PL/feat/piece of gear/Reaper Enhancement/etc. to min/max their build would fail. Even veteran players with most of the investment to build out their toon complete would not succeed in the most difficulty settings. FWIW... this is what we argued Reaper should be. I would be OK with DCs being balanced this way too if it weren't for other items in this list.
2. DC ranged vary greatly from class to class AND spell school to spell school. Compare the max necro DCs of a Wizard to those of a Cleric. Or max Enchantment DCs vs max Evocation or max Necromancy. Yes, some of the spells use a different save (fort vs will vs reflex) however there is some crossover. Which class and spell school should the DCs be balanced against?
3. The content difficulty settings make this task nearly impossible to balance on a D20... even in Reaper. This is one of the reasons we asked for Reaper to simply be Reaper without the 1-10 levels. The dev team was struggling to balance DCs across ELITE/HARD/NORMAL and yet now have an additional 10 levels to spread DCs out over. Even if you increase the save stat by 1 for each Reaper level and balance Reaper 10 against the max necro DC for a success rate of only 5%... the caster will gain an additional 5% on the roll of the d20 for every Reaper level below it. Add in the gap to ELITE and suddenly ELITE is no-fail every time for the players built to DC cast in Reaper 10.
4. As alluded to in point 3... this leaves us in a place many of us worried we would eventually wind up... the game being balanced against Reaper. If you build the DC system around ensuring effective DC casting in Reaper, then the other difficulties become trivialized. If you don't ensure people can DC cast in Reaper, then you'll see thousands of threads complaining that Reaper is melee only. Once again, the cat is out of the bag but this is why Reaper should have just been Reaper and the challenge mode impossible for everyone which we were promised.
5. Spell Penetration has the same D20 issue that DC casting has, plus... not every mob has spell resistance. See all of the above points, however, and apply them to the same issues inflated stats bring to a D20 system.
6. Old content. Not all mobs are built/balanced the same way. To really revamp spell DCs you will have to adjust mobs throughout the entire game. Probably not going to happen. I suppose you could build mobs in new quests with insane and all-but-impossible saves, but that still leaves the old content as easy DC killing fields.
7. New loot. Whenever new loot allows a DC caster additional gains to their primary casting stat or an extra +1 DC... if Reaper 10 was previously balanced to 5% with max DCs then it's now 10% with max DCs. As power creep continues to trickle into the game... those numbers will continue to rise. Eventually, we wind up in the same situation with DCs with all the previous work wasted.
So what are some possible solutions? The devs have tried many things throughout the years including:
- Immunity to insta-kills.
- Mobs with inflated stats which need to be energy drained before insta-killed.
- Divine caster mobs who cast mass death ward.
Each of these has had varying degrees of positive reception (although immunity is a lazy mechanic IMO) and have been somewhat successful. What, however, if Reaper mode simply included a behind-the-scenes roll straight up against a d20? Call it a Reaper Spell Check or whatever. The player can never boost their roll and the mobs are assigned a Reaper Spell Resistance stat of 1 - 10 (based on the level of Reaper) and whenever the player attempts to cast a non-damage spell against the mob they must roll higher than the Reaper Spell Resistance number or else the a red shield will pop up and the mob will resist the spell. At R1 you would have a 95% chance (followed by the typical spell pen and DC check) to succeed and at R10 you would have a 50% chance (followed by the typical spell pen and DC check) to succeed.
This would make the D20 relevant again and would ensure casters never reach a "no fail" point while still allowing for greater success rates for those who min/max their DC caster builds. It doesn't address the underlying problems ELITE/HARD/NORMAL had/have but... Reaper is the new normal?