The two likely outcomes if Luffy fails his block are the new crew member emerges or Doflamingo presents Luffy with such a deal as you propose, cross. In my opinion, Rebecca still has the best chance at eating the Mera Mera, whether she succeeds in advancing and winning the whole deal (holy crap if she does!) or simply her vengeful, though subtly hinted at traumatic backstory forces her hand to eat it out of desperation and against the rules. I personally like the latter because it would then force Burgess, Doflamingo and the entirety of Dressrosa and the gladiators to attack Rebecca, which would then prompt Luffy and crew to defend her because of the relationship or kinship the two parties have created.

Doflamingo's deal would pit Luffy in a horrible situation: abandon his brother's will or break his one attitude that rivals his ambition to be Pirate King--friendship. Essentially that is what Luffy views his and Law's alliance, so the inner battle Luffy would face is both tremendously cruel and intriguing. Obviously it would end with Luffy kicking Doflamingo's ass, but I'd sooner see Luffy beat Burgess fairly if Rebecca's story idea doesn't pan out and have Law defeat Doflamingo. Shake it up a bit in who faces the big baddy this arc.