Alright, I haven't got time to respond, so I'll do it now.
I think that I won't change spell level, but instead I'll lenghten the time in which you can use True Ressurection. I'm still thinking if dead creature can be brought back via Wish after that spell.
Try Unnaming in the Tome of Magic. That's Sor/Wiz 9 for a similar effect.
Crap, I knew there's such spell in a book I don't have...