I think they really need to either throw in some more open world content, boss rushes, or challenges that are unattached to the weekly gear grind that make use of the game's combat. We've got a lot of recyclable content in crafting/gathering/housing, chocobo racing, triple triad etc but none of it is related to actually making use of your gear for something.

Gear shouldn't just be used to make clearing already cleared content faster (or making the next patch's gear cycle trivial), instead, it should be used for things that are unrelated to those methods. Such as mentioned, Soloable boss rushes, challenge modes... But then why not some new ideas? Dynasty Warriors styled arenas or something like Pokemon Tournaments (except more 1v5 randomized encounters), Monster Hunter fights (hard but short fights for crafting ingredients) etc. Lots of other games out there to look at on how to make recyclable game modes for combat that are actually for the sake of fun instead of reward.

Imo, content meant for grinding is almost never fun as it needs to be long enough to not to instantly gear up, it needs to be accessible so everyone can gear up properly.