I only make endless games for gamejams with short duration such as Ludum Dare cause it's easier to do. Just make some varying assets yet sufficient for a game , code in unlimited spawnings (enemies, items etc) then put them in a level with unlimited time.
wrote Currently, my main roadblock to enhancing the experience of my project is figuring out how to integrate endless narrative/story into endless games.
well there can be repeatable Quests that change every time you get it
True, I think the main issue there is if you really want the player to feel like they are exploring the narrative/story of the game, those quests will probably need to be quite sophisticated, and not obvious in repetition.
I think players are quite good at identifying repeating patterns in games.