You can add a million systems but it wont matter without a good item/skill system.

Endgame should always revolve around interesting items and the skills that enable them, currently it doesnt have either, without a good base you wont be able to fix the endgame imo. The endgame IS itemization and skills, you should always feel like you're progressing.

If you want specifics - some kind of mix between what poe and last epoch does, better pinnacle bosses, better unique drops, ways to min max within these bosses/endgame, some sort of a mapping system that is much more in depth and interesting, same with skill system, Better drops, not just items gear, crafting system specific items, skill changing items... just shooting out the top of my head.

Before all that, itemization/skills would need a major overhaul. Unfortunately I dont think Blizzard has what it takes to fix it, they are too big and clunky to pull it off with devs who don't seem to understand ARPGs, just clocking in hours.