Really is unthinkable at the moment but if worst was to happen I really don't know what I'd want.
A small part of me would be open towards hiring within, I know it didn't work too well with Tim but Poch seems to have installed his philosophy around the club from U18s upwards and so I'd like to think someone can step up (Ehiogu? John McDermott? Kieran McKenna?) and take over, although it seems like Poch and his staff's man management and not just coaching ability are what brings out the best in players and so it wouldn't necessarily work with someone else and it's a huge risk as all three have no Prem level (or equivalent) management experience.
Externally, a stand out option would be someone like Pellegrini who's got an impressive CV of winners medals and also knows the Prem but I'd probably prefer someone like Eddie Howe, I'm a big fan of his and I think a young manager would be more suited to this side.
Of the options from what I can think of, I think I'd go with Eddie Howe, although I really hope we don't need to make this choice any time soon.