One of the aspects of ToB that was criticized the most is its heavy linearity. I think that there is room for improvement and one could be to mod in the possibility to recruit Illasera.
How to do that? Well, the option I was thinking is that she somehow gets teleported too into the pocket plane, despite her death (that won't be more inconsistent than many of the twists in the saga) or right before the final blow (perhaps with a dialogue ad-hoc when she reachs 1 HPs). Then, how to manage the consequential situation is open to interpretations, tastes and modders opinions. She can propose herself, just to survive, or because she wants to jump on the bandwagon, or as a trickery because she plans to betray at some point, or because she came in the pocket plane like a ghosts and wants to be revived like Sarevok. Charname could be the one making the proposal. Sarevok could be suggesting to force upon her a geass like his, if she doesn't want to be killed. Or even the Solar!
Reasons for recruiting her are simple too: maybe Charname wants to get information and control over a possible powerful enemy, or he/she believes that Illasera is redeemable, or the latter offers something in exchange (e.g. she could be the one revealing how to defeat Yaga-Shura or the weaknesses of the other Five).
Further interactions could be developed between her and Melissan.