Here are the scenarios I foresee:
1. EON, not willing to admit defeat with their bold casting choice of Craig, continue on with BOND 22 with Craig in the role.
2. EON, desiring to stay with Craig, is told by Sony to let Craig go. Bitter negotiations ensue, stalling progress on any future Bond films for a number of years.
3. After failure, Daniel Craig politely "bows out" of the role (after being asked to leave by Sony, undoubtedly), and EON hires a safer choice, Hugh Jackman, and continues on with BOND 22.
I don't think it at all likely for Brosnan to be returning to the Bond role. It's just not a really feasible option. EON would have to come crawling back on hands and knees to him after hiring Craig. They'd look like total fools and would be humiliated. When Cubby went for Connery, it was because Connery had left of his own accord. Brosnan was tossed - to go after the thing you threw away is somewhat shameful. Even if pressured to toss Craig, I think they'd much rather go with a different, more popular actor than Brosnan. Hugh Jackman, or such. Not to mention, bringing Brosnan back for one film is somewhat problematic, because one he's gone, you're in the same "search-for-a-new-Bond" pickle you were last time. And then the obvious choice of Hugh Jackman will probably be so far gone in his career, he won't want to give Bond a thought.
Furthermore, why is it at all in Brosnan's interests to come back? He's done nothing but moan and complain about how awful it was being in the part since he's left, he's found a newfound artistic freedom and credibility as an actor, and is in a position to really escape any Bond "shadow" as a performer.
And while this is purely speculative on my part, I don't think Brosnan, while very popular in the role, has the sort of draw that bringing him back would really be all that big of an event. He's not where Connery was, where Connery was really the *one and only* name attached to the role, so that further success in the franchise almost necessitated his return. I'm not sure that heralding "PIERCE BROSNAN IS BACK" would really be that profitable.