From an actual armchair developer, for what it’s worth (nothing) - here’s just an idea I had. This system solves the issue of dividing the player base and having to keep a low number of difficulties as a result.
First off: if any difficulty differences with (1-60) and (endgame) have to do with enemy AI or CC duration/chance or anything like that, let’s remove them. Make within each segment, the most difficult behavior default. The only thing scaling with difficulty is ingoing/outgoing damage, and rewards, which both scale to the player.
Secondly: Yes I am aware that in my shared difficulty model, a player could choose a high difficulty and another could choose an easy one, and power level the first. “Exploits” like this already exist, and power leveling is a staple of the genre, so I don’t see a problem with this.
1-60
Two modes:
Campaign 1-60
Monster level increases along the critical path from 1-60; nearby zones have similar monster levels. Monsters below the character’s level provide substantially reduced XP and those above provide substantially increased XP, allowing the player to take as much or as little time as they want.
Three difficulties: let’s call them Normal, Nightmare, and Hell.
Higher difficulties reward more loot and gold, but not more XP.
Players on any difficulty can party or coexist in shared environments, as monsters scale to them individually (a la Borderlands 3 and current 1-60 D4 content, regarding monster level.)
Upon beating the campaign, the world transitions to:
Non-Campaign 1-60
This mode can be chosen at character creation if the campaign has been beaten. No campaign quests exist. Monsters scale to the player’s level. Other than that, the same rules apply as Campaign 1-60, except higher difficulties also provide more XP. Designed for quick leveling.
Once a player is at level 60 in Non-Campaign, Pit opens. Beating pit #whatever opens Endgame and Torment Tiers
Endgame
Functions pretty much as we know it now in regard to torment difficulties. Solo pit clears of specific levels unlock the corresponding torment tier. Players of any individually chosen torment tier can coexist in the same manner as explained above.
If armor and resistance reduction have to be exist, this is where it happens.