If it goes south, you have two characters that had no hopes of any relationship seeing a glimpse of hope for one crushed in the end, witch is good tragedy. IF it is a result of the final battle, it highlights a personal consecuence for the characters fighting the good fight. You cant come unscathet from something like this. If they have to choose between theyre love and being heroes, this whole thing takes on a new level of complexity.
Mind you, im just speculating. Just a couple of reasons why this romance could be VERY justified, mostly knowing Whedon´s work.
There is rumors that somebody might die. Same thing. I know many people dont agree with it, wich is fine, each has his or her own opinion, but i do agree with Whedon. If you say "there is no way we get all trough this" at the same time you are creating a situation where such a line is credible and you dont kill, at least, somebody, it does feel a bit like a cheat. We are used to it, but i dont think taht makes it right. You can do that once, but the secdond time, you should make that statement EARNED.
If you have an ultimate battle and you have someone die in it and two characters that are allmost touching a dream they never thought could be so near smashed, it makes that end battle more complex and real than just the heroes wining the day... again.
Of course, it all might not happen this way at all. But knowing Whedon, i doubt it wont. His Astonishing X men had two lovers reunite after he got "resurrected" so they could live trough the love story they were denied before just to have her sacrifice herself for the greater good and leaving HIM this time alone. Heck, even his run of Runaways had a tragic lovestory at its heart.
One might not agree with it nor like it, but Whedon has never really done something without a specific reason.
