The good thing is that GOTG is "Cosmic Marvel" so it doesn't really have to cross over with the other MCU films until Thanos makes his presence known. It basically goes Earth MCU (Iron Man, Cap, BW, Hawkeye, Hulk for now, BP) -> Human-y Magic Space Marvel (Thor, Hulk in the future, Dr Strange probably, Captain Marvel) -> Cosmic Marvel (GOTG, Dr. Strange probably, Captain Marvel probably). So there's essentially 3 tiers which intertwine, but also exist independent of each other. The post-credits scene from Thor 2 was the only real connection we got from GOTG to the established MCU, and that's completely okay. By the time Infinity War hits we'll have Thor, Captain Marvel, and Dr. Strange (as well as Thanos obviously) connecting the Earth MCU to Cosmic MCU.
Basically, there's no reason to have Iron Man or Captain America show up in GOTG because there are already strings connecting them via other properties. I would not be surprised if GOTG Vol. 2 featured some Dr. Strange and Thor elements to start connecting it even more to the others.