As you're (probably?) aware, I've been thinking about how to reform Nia and Melia's MUs as part of my post-XC3 Xeno JRPG MU charts, and...
...well, I hope you guys like cursed MUs:
Nia (XC3 powerset, XC2 continuity) VS Eli Ayase (Granblue colab powerset, Love Live SIP continuity)
- Both initially stood in opposition to the resolve of the main character, before being convinced by them in accordance with disagreements with the side they used to be a part of (Nia initially being unconvinced at Rex's wish for Elysium that could save the world, with Nia being ex-Torna before his death-turned revival with Pyra, and Eli looking to shoot down Honoka's attempts at being a school idol, convinced by mother Minami that it was unworkable for the then-dying Otonokizaka, only to work as a coach for the group to offset her own confidence issues). E: Both mother Minami and Torna Jin become sympathetic 'antagonists' towards the end (MM being the schools bureaucratic roadblock that made Eli come off as a bit of a wet wipe
and also leading to Kotori spearheading the worst quarter of SIP seriously **** THAT ARC, and Torna Jin wanting Amalthus dead despite his misanthropy).
- Both also come under pressure from their past (and present) regarding their home nations (Nia being a Gormotti
and also Welsh, while making friends with Morag despite the less than favourable relations between Gormott/Torigoth, Mor Ardain and Uraya, and Eli's family being... well, part Russian blood). For this, Nia gains her Flesh Eater form due to memories of her sister being 'tied to her', and Eli has her hangups regarding her alleged lack of ballet talent, despite using it to choreograph Muse.
- If you want to include it, both also have partners in their lives that act as a contrasting point of view (Dromarch being Nia's go-to Blade for the majority of story fights, as well as being a smooth-spoken butler (despite being a tiger, but there's an inside joke in here somewhere), and Nozomi, being Muse's 'guardian angel' and (at least I'd argue) master manipulator to nudge it's individual members in the right direction).
- If you want to bring any of the 'continuities' from outside their initial origin into this (XC3 pun only slightly intended), then yes, both would be from 'other worlds' (Aionios is technically a neutral planet, hence the XC1/XC2 worlds combined).
I guess to end off, I'll mention that I know dick about Grablue, so splitting the 3rd years profile that we have between characters (and who gets what for powers) would be nice. Nia more than likely wins if we go with Z/Origin scaling, but Eli probably wins if you want to stick to T6 scaling (/Aion being only planetary imo)?
3rd years allegedly have status res, so no cancer/cell manip for Nia, IF you went with T5/T6 keys.