On the one hand, Ra’s was killed earlier in Season 4.
Judging by how Barbara was reeling from pain in her head, the fact that nobody else could see Ra’s, and the twist when his face melded with her own, I’m guessing that Ra’s somehow transferred his consciousness into Barbara in the event that Bruce did indeed kill him.
Even if villains like Jerome and Butch were able to come back from the dead in their own bodies, something tells me Ra’s’ return won’t be simple.
That doesn’t answer the question of how Alexander Siddig will reprise the role after Ra’s was physically killed, but we can bet that Gotham will find a wonderfully weird way to do it.