Sorry, Thanos will most definitely be a Big Fig. I suppose I can see them doing a smaller version like they did with Hulk but I'm doubtful.
LEGO does need to redesign their big figs, nobody seems to really like how they look and the posability is shit.
There are a few that were great like the Rancor but that's an example of LEGO putting in the extra effort to make it work. If only they would put so much effort into each big fig instead of having them all look like carbon copies locked in the same pose.
It's always a shame when LEGO releases new figure types (friends/disney princess/big figs such as Hulk) where they throw posing options out the window. The original LEGO minifigure design was great for its time. While it has its limitations, you can get some pretty good movement out of it while still having tons of interchangeable parts.
Most of their subsequent figure designs have not put as much thought into these strengths. One of my biggest LEGO complaints. I want to see more modularability not less.
Imagine if you could had the same modularity in the big figs as in the classic minifigs, you could take your hulk big fig and swap in some parts to make him into a World War Hulk big fig, as an example. But nah, you're stuck with that hulk for the most part.