Okay I don't claim to be an exspert so if I make mistakes please forgive me.
So besides hating the idea of a Marvel universe without Iron man even If it's a minimal presence I think Tony Stark will have quite a big role to play in infinity wars. This is based on the infinity gauntlet story line, which I believe contains Iron man collecting all the Infinity Gems (stones) and putting it in the gauntlet going a little nuts with power.
This is a little simpler. The infinity stones are found spread across time, space and realms. So it wouldn't hurt to have some one who can travel across realms. That and its comfirmed here. http://www.cinemablend.com/m/new/Thor-Definitely-3-Upcoming-Marvel-Movies-70891.html
Even simpler. Cap is training the new Avengers. So unless something major happens in civil war. He will be present in some way.
In the end they have been building to this since the tesseract was introduced in Captain America winter soldier. After all that build up, I would be disappointed if the original cinematic group wheren't involved in the last film.