Re: Thanos: Mastermind of the MCU
He definitely plays a role in GOTG; Feige has confirmed it. And that's always been obvious.
And I don't see how TDW could sweep him under the rug, since the movie will likely have to address his existence from the very start. Loki's going to be on trial, and he's going to have to answer the question that Thor posed to him in TA1: "Who gave you this knowledge/power?" And there's that whole thing about Thanos' threat/promise to hunt Loki down.
AoU....I no longer believe Thanos will have any presence in TA2 at all. It's still possible, but a lot more remote. Ultron is a terrestrial threat.
After AoU....who knows. I hope GOTG becomes so hugely popular that Marvel will be forced to re-up for a Phase III sequel, and you'd certainly expect to see him come back for that.
THE COTTON AVENGERS
...They move like slick cotton on oil.