I'll bring this back to the OP. There are plenty of posts from uber-players in this thread. There are also a relatively small number from newer / more casual players posting in this thread. A significant percentage of uber-players are forthright about what they want; they want dungeon scaling eliminated entirely or eliminated for elite difficulty.
"It's a group game" gets repeatedly trotted out as a reason for why making soloing fun should be a lower priority. However, these kinds of suggestions imply a "zero-sum" approach to setting difficulty, and they answer the wrong question. The question is how can you make the game better for groups of all sizes, and not just for larger teams, at the expense of smaller teams (or soloists). And, on that note, multiple posts in this thread have pointed out that even on normal difficulty soloing can (already, under the current system) be very challenging for those who are less experienced.
On another point, at some level of experience, wealth, and skill, players will find that a random stranger being added to the team isn't likely to make missions easier. At some point on the experience / wealth / skill curve, soloing or working with close friends who are near the same level becomes the most efficient option, as well as providing insurance against some of the "low points" that can arise when teaming with strangers (teammates who are not only mistake-prone, but who don't listen, etc.) If the devs focus on catering to uber-players who are chasing the nigh impossible dream of turning teaming with random strangers into the "most efficient" option for anything other than raids, then they're probably going to screw things up for the other 99% of the playerbase who are not uber. Since it's the uber-players who are by far the most frequent posters on the forums, I don't envy the devs reading through this thread the challenge of figuring out how to set mission difficulty.
If you really want to create a game that's enjoyable for teams of all sizes, then keep dungeon scaling at all difficulty levels. Find ways to tweak the current system, instead of making a radical change that cuts soloists off from attaining a fair number of relatively attractive favor rewards and more generally narrows the range of ways in which they're likely to be able to enjoy the game. A few specific suggestions:
1. "Flawless" bonuses can be calculated "per player" instead of "per team", and be larger for larger teams; thus, one player doesn't cost the entire team 10% of their bonus by making a mistake (and get treated badly / squelched for it, sometimes), and more experienced players still have some incentive to help newer players learn how to avoid dying. The larger flawless bonus for larger teams helps to compensate for how much harder it is to avoid any deaths on a team. It also reduces the incentive of team leaders to set arbitrary limits for hit points for teammates, because less is at stake if a teammate turns out to be more fragile / less skillful than hoped. Average team size would be likely to increase, with this change.
2. The scaling value can go up at a decreasing rate, for each increment in team size, and be very low for the last (and perhaps second last) team slot, to give teams a stronger incentive to take a chance on whoever is willing to join for that last one or two team slots. That helps newer players secure a place on teams with more experienced players, so they can learn and get better. This would also increase average team size.
3. Experiment with different ways of scaling up mobs, including different mixtures of scaled up mob hit points, scaled up mob damage, and scaled up mob numbers -- be careful about this last one, though, as this favors teams loaded with AOE attacks, and hurts many melee characters.
4. Finally, provide more randomization in dungeons, and increase randomization as the difficulty level increases; the game becomes far easier when a player knows what's up ahead, so make that less of an issue. This change would also tilt incentives further in favor of teaming, for many missions, by getting uber-players who might otherwise level up by soloing on elite (which they can probably do for many missions while practically asleep, because they know what's ahead) to team up to take on the randomized challenges up ahead.