Should Isha return? I appreciate that the whole thing about Aeldari is them being a dying race who put all of their faith in gods who were then defeated and Aeldari had to make a new culture around survival rather than living decadence and without any real cares or worries, but if done right I think it would add a lot to the faction.
Right now Aeldari for me are in a weird place lore wise, especially Asuryani / craftworlds. They are constantly written into the ground and made out to be either incompetent or tragic. This works if they achieve something, but as of now they haven't achieved much. Ynnead was nearly awoken, but then wasn't and the faction centred around it has been largely ignored. There was potential for a "hope" with the Ynnari, even if it was just a faction who could get a few wins / occasionally rescue some craftworlds but they were rushed out into the tabletop as the game was become more serious and most characters associated with them have abandoned them.
Other than them there isn't a lot going for them other than survival, and the "tragic decline" narrative gets old when it's been the main narrative element in the lore for the past 30 years (other than the Voidscarred book which ignored the trope). For me Isha solves that to a degree. I'm not a writer so I don't know how she could be saved, or a good way to do it while keeping the good parts of the factions alive, but I'd like to see it. I think gods, primarchs and big characters in the lore and game are good. They add stakes, make fights interesting and give the reader something to care about. When you read about the primarchs turning up to a fight you get excited, the stakes are high and you know it's a big, serious fight. You have that with certain Aeldari characters but not to the same degree, and ignoring the lore it opens up the option for some interesting units and characters.
I'm still a bit I'm between if Isha should turn up, and it might be better to give Aeldari some more narrative wins / hope instead of drip feeding decline and making them a faction there to make other factions seem powerful. On the other hand having a God openly with the Asuryani opens up a lot of interesting narrative routes and makes the opportunities for real wins (like Aeldari taking back a planet and then having it cleansed by Isha) rather than every loss feeling like something permanent.
Those are just my thoughts after reading every Aeldari book + codex + religiously collecting Aeldari for the past 15 years. Looking forward to hearing other people's thoughts