Thanos

I presume he is not showing up until the Infinity War movies. After watching GotG again on the weekend, I feel like without much screen time in the lead up movies he might be underwhelming.

At this point, and going from memory here so I might be forgetting things, it is mentioned he is a Mad Titan and likes to call people "Boy". When Ronin said "Fuck you, I'm keeping the stone" he didn't really do anything about it.

And then at the end of Age of Ultron (I think) he says he will do it himself while picking up the Glove.

Nothing in any of this really suggests he is scary/capable/a badass. He just sits on his asteroid chair and lets people talk shit to him and after the failings of other people, he finally says he will do something.