I can understand why having more bloody adventurers around would somewhat undermine your adventure when you know other people are doing the exact same shit as you. Also their fucking annoying in wow when they mine all the ores, harvest all the herbs and kill all the wandering beasts first.
Also the quests cant really be all that unique, like oblivion's thieves guild quest which results in you stealing one of the elder scrolls wouldn't make much sense if 100 other people also did it after you.
but other than those two I don't see why else the game would be that bad. The perspective you play in should really be first person because no idiot played in third person for morrowind, oblivion or skyrim apart from idiots who wanted to see their asses. And the combat should really be like the past three games, you click you attack, not an auto attack with fucking abilities Bar like WoW.
Although maybe having an auto attack would be fine since even in morrowind some enemys took fucking multiple sword hits, like about 50 and it can get tiring but it should be optional.
Of course you cant pause to look into your inventory or change spells but that just means more reason to have potion and magic quick selection.
And if they can actually make the world too feel real and alive rather than just some game world would be important for immersion, WoW may have been better if nature had a few less fucking enormous trees but instead a lot more small trees and bushes rather than a flat landscape.