I don't think there's anything wrong with just "2" or "II."
Spider-Man 2 worked fine. The Fantastic Four sequel NEEDED a subtitle because you can't call it "Fantastic Four 2." X2: X-Men United was kind of crappy title for a good movie (I would've preferred X2: Mutants United), Batman Begins 2 wouldn't have worked, Hellboy II: The Golden Army works in spite of itself. Blade II worked.
Honestly, I think where you can afford to just use "2" or "II", you should. It's simple. Subtitles scream "direct-to-DVD."
The exception to the "2" rule is if you can come up with something legitimately creative, a la, The Dark Knight, which isn't to say Iron Man should go that route exactly, because I want "Iron Man" in the title. But "The Invincible Iron Man" or "Iron Man: Mark II" are pretty sweet. My only issue with those are that IIM is already that animated movie, and "Mark II" refers specifically to a suit we already saw in Iron Man.
But hey, I'm not too much of a stickler. I'd be happy with Iron Man 2, Iron Man II, Iron Man 2.0, Iron Man: Mark II, Iron Man: Mark 2, The Invincible Iron Man... just no subtitles, unless they can come up with something awesome.
Absolutely no "Rise of"s.