Regarding the general direction of the MCU over the course of Phase 3, I think I know what they're trying to do, and it would be awesome if I'm right. The Avengers are split in half (with one member potentially out of commission for good). The Hulk is MIA. Thor is going to discover that Loki is ruling Asgard, plunging that realm into chaos. Nothing is ever normal for Doctor Strange, so he's going to be constantly distracted. The Guardians of the Galaxy aren't in a position to do much because Earth isn't even on their radar, and the sequel might do something to incapacitate the Nova Corps.
The movies are systematically making Earth less and less ready for Thanos, while simultaneously deconstructing any back-up they may call for. It's a slow-building march of doom, and when the **** hits the fan, it's going to be ugly.
Also, in the first part of Infinity War, there needs to be a scene where Thanos kicks the ever-loving **** out of the Hulk. When all the machinations and bodyguards have failed, the Hulk has been able to beat down both Loki and Ultron with ease. If you want Thanos to be a threat, the quickest way is to show that "Hulk smash" is no longer a viable plan.