Okay do not give up this quickly, some of the plot points will have to be adjusted to better fit with canon, but that is the last reason to give up. I am here with as many suggestions as you will ever need, you just have to actually use some of them. I am here to help.
So I am going to start listing off ideas based on what I know in order:
1. You are going to have to give up on Iri (Physically at least) but if you are planning a grail war six I have an interpretation of Kerry's backstory that might let you get him in. Let me know if you are interested.
2. Dying Illya, Altria is still there right? So Avalon still works, stick it in Illya. With no heroes in the grail to degrade her body the sheath should fix her up well enough that she won't need it anymore by the time it is needed for anything else. No adult Illya though.
More to come.