Scrap any heroic / epic tiering.
Scrap any kind of dungeon scaling with party size.
(Casters don't need dungeons to be easier when there's less party members, the quests become easier because they don't have to hold others' hands throughout the quest)
6 tiers for all quests: casual - normal - hard - elite - (epic) - insane
(I would suggest finding a different name for (epic) but I couldn't think of a good one.)
For quests from old content like desert and Von the (epic) level could be what "Epic" was before u14, with the insane being a cr25-30 setting that is quite a bit more difficult and is there as something that might still challenge even a group of capped level 25s.
For new quests in forgotten realms, elite might be a cr25 type of thing which is actuallly very difficult, (epic) might be a very tough cr 30, and insane might be a "cr40 don't even try this with all capped level 25's with maxed ED's because you still probably won't succeed after using 50 mana pots and breaking all your equipment". (Also hopefully insanely harder not just due to more hp for and damage from mobs but rather due to some new "twists" in the encounters.)
Anyway, it would get rid of the potential double-scaling issues and "why is epic normal easier than heroic elite?" stuff we are currently seeing.
Note: challenges might still benefit from some form of party scaling, but that's a separate issue.
(For the older content that previously had "epics", the epic shards/seals/scrolls might only drop on the highest 2 difficulties.)