Honestly, as much as I hate to say this, I think we need to look to WoW again.

Blizzard is (in)famous for taking ideas from other games and then improving on them to make them their own. The new Legion expansion has taken relic weapons from FFXIV and turned them into a new character progression system. There are multiple relics per class, unique stories for each, and the weapon can be levelled, upgraded and enhanced as a new form of post-cap character progression. If SE wants a guide for what to do then they should look at how much better Blizzard is now doing relic weapons as that system alone would likely satisfy most of the players who currently feel like progression stagnates at the cap.

My main point though is that the game needs a form of endgame progression that is NOT based on simply getting new equipment.