It'd be nice if they never include characters you can actually kill though, unless they took that extra step and locked out characters you can by simply accounting for those in the Keep.
They've done it for Isabela, for whatever reason that might be. Story wise, I have absolutely no idea why she would join. She only joined up with Hawke to benefit herself. Stayed for Hawke and Co. but regardless, she is selfish when it comes to things like this. I can see no reason why she would become an Inquisition agent. Hopefully, BioWare clear that up but if not, then I see no reason why they can't bring in dead characters.