You know, The Avengers movie is a ways away, and still has alot of ground to cover for the story to play out, and in the mean time, I've never seen two movies that had as much core content as Ironman and Hulk. From S.H.I.E.L.D. playing a role in both movies, and Tony Stark's cameo at the end of the Hulk -- Just great stuff! Now that both movies might have sequels, here is my argument on why Marvel should combine them...
1. Neither heroes have that great of a line up in arch-villians. I will give Ironman the Mandarin, but other than that, most of Hulk's and Ironman's best fights come from being involved with villians from other series.
2. It goes in continuity with both movies, considering all the crossover elements, and the situation with the Hulk being a rogue, and Tony Stark hot off his "I'm Ironman" speech, you can almost pick up the movie right there.
3. Hulk-buster suit.. nuff said.
4. It will please the hardcore fans, think about it, "Hulk vs. Iron man, in theaters may 2011" how could you not get exited to hear that!!!
5. Most importantly, IT WILL SAVE MONEY, AND MAKE MONEY!!! Marvel is going to dish out a lot of dough for the individual sequels, but if you combine them you already have two widely popular characters in one, plus all the money needed to keep the actors they want (cough: sam jackson) in certain roles, plus if you could get Ed Norton back, and put him in the same movie as Robert Downey jr you have star power on that alone. It just makes too much damn sense...