I can imagine 4 different scenarios.
1 - Law is captured, RP stolen.
2 - Cross Guild free him. 4 out of 7 Shichibukai assembled in one crew. Boa to follow, Doffy to be freed and last spot Weevil or Moria.
3 - Sengoku saves him and dies. Buddha fruit goes to ...? Coby was a great guess above.
4 - Sword saves him. Finally revealed that Law is a vice admiral of Sword. Other prominent ones being Kuzan, Fujitora, Garp, Sengoku, Tsuru, Drake, Coby.
Knowing Oda it is probably option 5 not listed above.