While Romney is the putative front-runner and there is still plenty of smart money on him, he hasn't actually won anything yet.
Let's hit some overall trends. Ayotte is from Romney's region. Geographic balance isn't an absolute necessity (see Clinton-Gore, two southerners) but New England is not an electoral power-house. Ayotte was elected in 2010 - she is probably a lot more seasoned then Palin, but she is still relatively inexperienced. Although Romney was born into a political family, he hasn't spent any time in DC - so he is still an outsider. Plus he needs someone with rock-solid conservative credentials to shore up party suspicions that he is really a moderate.
So he needs a southern conservative with DC experience. There are any number of possibilities to fit that bill - Jon Kyl and Lamar Alexander leap to mind. (Marco Rubio does not.)
One interesting character who fits it perfectly is actually Newt Gingrich - but something tells me that he isn't terribly interested in the number two slot. Gingrich is a brilliant idea machine, but even if he were interested, would number two be a good fit?
Speaking of which, I answered the Politico Arena question of the day:
Will immigration stance hurt or help Newt Gingrich?
It is likely that Newt's stance on immigration will hurt him with "the base." Fortunately for him his major rival has a number of weaknesses with the base as well. Part of the problem is that this base has calcified into a set of impossibly rigid positions that no candidate can realistically satisfy.
However, this position will serve Newt well if he can make it to the general election as it highlights him as an independent thinker and it reflects a more humane side to a Republican Party that is looking increasingly mean-spirited.