Tampa has a bit over $48.5M in contracts next season. That's with 6 forwards, 6 defensemen and the two goalies. Add $10M for Stamkos, $2.5M (minimum QO) for Killorn, $5M for Kucherov and you are at $66M and still need 3 more forwards and a 7th D. You'd want to re-sign Brown and Namestnikov, and at about half a point per game, that'll cost you another $4M+. You also want to re-sign Nesterov since you're likely losing Coburn (which means Nesterov is in your top six with Koekkoek as you 7th) -- that's going to be close to another $2M. So you are probably already at the cap, and still need another forward. And I was probably a bit too optimistic with some of those salaries.
They have to lose/move one of Stamkos, Callahan or Bishop. Callahan's contract is a boat anchor. They don't want to lose Stamkos for nothing, and with Vasilevski and Gudlevskis they likely have the most depth in goal.
On the other hand, I suppose they could buy out Callahan, but that's a lot of money for a *loong* time.
So Bishop is plan A, but I'm prepared to go with a Driedger/Hammond tandem for one season if need be. If it doesn't work out there's always the free-agent market or trade.