I really want them each to feature in each other's movie in some way, even if it's just a cameo. Maybe in Cap 2, there could be some flashbacks between WWII!Howard and Modern Day Tony, showcasing Cap's relationship with both...

I would LOVE LOVE LOVE Extremis and the Mandarin to factor into Iron Man 3. Seeing RDJ's Tony undergo the Extremis transformation would pretty much make me happy forever, and the Mandarin is a great foil for him.