I think it would be kinda neat if there were some options to influence Eryka a bit, either through transformatives, corruption, or dialogue.
I'd also personally really like it if she became a companion, seeing as it seems like she's on the title screen of the game with Kas, but at the same time I can understand this not being a thing due to the amount of companions currently available and planning and so on.
If I had ultimate control on her status I'd potentially have her set up in a similar system as Azzy: You've got her normal form that's a white mage/warrior hybrid focused on being a healing tank for the party, and then you give her a corrupt charm of some kind (Maybe the Waystone Charm that Kas gives you initially, if you were buddy-buddy enough with Kas to have her infuse more energy into it?) and partially transform her into a demon in order for her to switch gears into a full offense party member by building on the corruption that Kas already touched her with?
Though this also has some issues with being pretty similar to Arona's combat builds. Hm. I'm sure something could be done, but this whole thing is a flight of fancy built upon the extremely slim chance that she becomes a companion, which I doubt is going to happen, so eh.
EDIT: I stand corrected Eryka being on the title screen.