It really does depend on where the film sequences are in relation to how they're perceived.
From the trailers and teasers, we can calculate that the encounter between Thanos & Steve takes place toward the end of the film, and that the scene is taking place in or around Wakanda. Thanos is clearly shown to possess only two stones - likely, at this time, to be going after the Mind Stone from Vision. In the scene, and in fact, all scenes, he's only shown to have the Power & Space Stones.
The Power Stone should make Thanos tougher, the Space Stone should give Thanos the ability to travel across the universe. If (come that scene) he already has the Reality Stone, then he's already (presumably) powerful enough to start changing things to his liking; such as impacting the Avengers with a mere thought (if he has the Reality Stone, he can turn them all into Mice, right?)
Assuming Thanos only has two stones at that point, and assuming he defeats Steve, Wanda, and Vision, he's still only going to have three stones. He's still going to have to get the Time Stone from Stephen, then travel to wherever the Collector is to acquire that one, and then to travel further to wherever the Soul Stone is - he'd have to collect the last four stones in an awfully short amount of time, which I just don't think is feasible, especially as the Soul Stone is going to need a bit of background before Thanos simply picks it up!