We all know that Big Mom flirted with the idea that the Straw Hats might lose some of their friends in a worst case scenario, and we know how freakishly strong Big Mom is compared to Doflamingo, who could've easily killed the entire city in a few seconds if he wasn't so melodramatic with his bird cage game. So what if Jimbei will die in the Whole Cake Island arc? We don't yet know what he lost to the roulette, maybe it was a vital organ that allows him to just barely survive long enough to help Luffy and Nami escape, maybe a little longer as well. But Luffy already asked Jimbei to join, and if he's not going to be the next Straw Hat, then the only reason would be that he would die. And I don't blame Oda if that's the case. Jimbei has already played a vital role in the One Piece story - he comforted Luffy after he lost Ace, and reminded him of what he still had. Also, he doesn't really fit a unique role in the Straw Hat crew. Some people have been thinking about him as a diplomat for the new world, which is cool and all, but it's not really a unique pirate-like role I think. ("Parley? I hate parley!") On the other hand, Ceasar could be one of those "never believed he could be redeemed but Luffy is so awesome" kind of people. In fact, Robin's feminine beauty was surely the only thing that allowed us to accept her more when she wanted to join. But Luffy doesn't see that. If people wish to redeem themselves for real, he will rise to the occasion.