I doubt Ace will join the crew unless something happens to Whitebeard. Even then it's more likely that Ace will start his own pirate crew and be Luffy's rival.
I though this through… If ace is alive i can imagine him return to WB defeated and either:
- returns defeated and WB dismisses him (this is unlikely to happen because WB crew members are as he says "his sons")
- Ace reports to WB explains to him his powers and that he is after luffy and is allowed to join the sh (as a honorary member or whatever :lol:)
but this is oda the possibilities are inumerous...
EDIT: Gorlom's post!! I forgot bb told ace hes after luffy