Board Thread:General Discussion/@comment-11588183-20150501071246/@comment-733050-20150503224713

Well FortressMaximus pointed many fans have theorized it. In any case Marvel will likely introduce the Soul and Time Stones in Doctor Strange, GOTG2 and/or Ragnarok, unless they leave one to Infinity War Part 1. There's precedents for time travel in Captain America and Black Panther stories but I think Marvel wants to keep them grounded, hence why stones weren't introduced in Iron Man 3 or Winter Soldier.