I'm sure the on-screen chemistry is going to be amazing! And I agree that RDJ is aware of slash and that people enjoy slashing character he portrays, and will hopefully ham it up for us. I'm sure Chris will be happy to go along with it. Maybe we'll get Steve/Tony as a canon pairing in Avengers 2 or 3 or whatever, if we're super lucky. I mean, I only know some of Joss's other work, but considering the size of the Spike/Xander fanbase among Buffy fans, considering Willow was the one canonically in a same-sex relationship (apparently, either Willow or Xander was planned to be gay, they'd included a whole bunch of subtext with Xander and several of the male characters, and then Willow suddenly started dating Tara... which I don't overwhelmingly mind, because Tara was awesome, but was the victim of Joss's unfortunate tendency to kill off important characters if people don't stop him, and anyways Xander slashy subtext like whoa, in short, I'm hoping we get as much subtext with Steve/Tony as we did with Xander/everyone way back when, or even better canon ship).