I really like his unarmored appearance for IW.
I know some people think it’s a bit boring, but it serves multiple purpose:
1. Thanos is comfortable enough going into battle with his casual clothes because he doesn’t believe anything will really legitimately threaten him or his ambitions right now. That he’s being so casual about fighting the Avenger’s and Earth’s armed forces is very menacing in and of itself.
2. They get to save his armored appearance for later, when they feels he needs it. Be it against an actual threat or to make a statement. I’m happier not seeing everything at once.
3. It seems clear that they want to dedicate a good amount of IW to establishing Thanos’s backstory, motivations, personality, etc. The almost regular clothes serves to humanize him and make him almost relatable by appearance alone. “Okay, so he’s not just a dude that sits in a hovering rock chair all day in armor. He wears normal clothes sometimes to run errands that leads to wiping out half the universe. Cool.”