Sadly when I look at the drivers available I cannot see anyone I would replace Kimi with and the two you have mentioned in Bottas and Hulkenburg; Hulkenburg deserves a chance in a good car but I don't think he will get the chance with Ferrari; for me Mercedes is more likely to give Hulkenburg a chance if any of the top teams.
Bottas, I find hasn't nipped Massa enough or by a big enough margin to prove that he is good enough for a bigger team like Ferrari. For commercial reason taking Bottas makes sense but does Ferrari really care about marketing the drivers for commercial reason? I don't think they car as the team means more then the driver (In most teams; if not all)
For me it comes down to Who is going to be the best second driver (team mate) to Vettel will get the seat beside him so that rules out in my mind: Rosberg, Button, Alonso, Grosjean
It leaves most the rest of the field other then Red Bull drivers with the two above being most likely due to speed and experience as I don't see anyone else good enough.
Anyone in another series maybe that could fill that role?
There is no way Ferrari would bring in a 'rookie' to the team, as in someone from outside F1, unless it was someone returning to F1, but that isn't going to happen either. The only driver probably capable still of being able to return would be Webber, and he wouldn't go back to being Seb's bitch again.
The only other driver I can think of that's shown enough pace to be considered, and might be available soonish is Sainz.
Vandoorne has the speed, but he's going to McLaren, Werhlein is next in line for Merc, RB has their pairing for the next couple of seasons, and there isn't anyone outside of Bottas, Hulk and Sainz that I'd even consider.
I think Red Bull is trying to keep Sainz but loaning him to Renault starting in 2018 when Renault will become a stronger outfit and that would put Ricciardo into his 30 so might be primed to leave Red Bull. Sainz would only be 23 by then so they could afford to loan him longer too!!