Well, think about it. Keeping Thanos behind the scenes for a while makes perfect sense. We've only seen him so fat for what, 15 seconds? He surely still has a lot of scheming to do before he pops up for a big climactic battle. Him not being the main villain doesn't rule out his appearance, anyways. He could still show up in TTDW, GotG, TA2, and anything else, really.
Also, this is in no way confirmed. Just Jim Starlin's interpretation.