You can squeeze WWH into the third movie along with Thanos in the final act. I think the buildup to the Infinite Guantlet and Thanos will happen in Thor 2, GotG, and other solo films. I don't think Thanos will be the main villain in Avengers 2 although that remains to be seen. If Thanos is the main villain in Avengers 2, he very well could remain so in Avengers 3, but if not, the only other comparable threat would be a WWH scenario. After an alien invasion and Thanos, I don't know if you can top that. Ultron and Kang do not fall on his level.