It would be kind of cool to have a Thanos/Galactus style event featuring Thanos with the final reward being a Thanos 5* cover. First boss fight with no infinity stones, but he gains stones with new rounds, adding abilities as he goes. They have mechanisms now for using more than 4 powers, so they could figure out a way to use 6 stones for 6 powers. Just a thought.
I fully think they’re planning this for around the time Infinity War rolls around. If the game is still around by then. Perhaps in the meantime maybe a Civil War event to alongside the new Cap movie.
They’ve had events to tie in with the last Avengers movie and Antman, so I think you can happily presume there will be more movie tie ins as they get pumped out.
They seem not to want to make boss-type villains playable characters anymore. Ultron and Galactus are obvious examples, and they said that Apocalypse would be more likely to be a boss character as well. I’d expect Thanos to fall in that category.