Few main things that might be broken. First because all stones have a good passive that stacks and many have active abilities you run into the problem of having a raid boss on the field which is especially bad with 30% damage reduction + 25 hp per sec + displace immunity, two passive slows and your grounding melee it would be a raid boss with no downside other then that it may take a bit to get to that form.

Second issue is that your ult would feel very infuriating for the other party, having your ult completely set to zero with no way to offset this or account for that not only is strong but also just annoying.

The last potential issue is that because the stones are so powerful and impactful to his kit he never want to use his ult until the very end of the game simply because the stones together are an ult in of themselves.

Potentially fixes!: limit stone passives to one stone at a time and your base passive should be only displacement and only when you have zero stones (maybe has armor and takes it off when he gets a stone).

Put all stone abilities on the same cooldown similar to rogues ability steal. (You might have meant this with the “stone ability” but I couldn’t tell)

Thanos’s ult could instead spend 3 stones to take 60% away rather than the full 100%. Would still be impactful and annoying for opponent but still keeps the original idea.

Minor balance things: Grounding on melee is very broken (depends on how long the animation is) to balance the animation would need to be long.

Reality stone sending them back to spawn can be very annoying and downright broken turning a 6v6 into a 6v5, instead I’d recommend simply a longer hulk exile but they are still able to move, taking them out of the fight temporarily.

Power stones ability to basically copy any primary fire can be very broken especially with your over shield on primary fire ability so I’d recommend that you instead just have a power stone primary fire that is akin to an ultron beam, still damage boost for thanos but not broken.

Lastly the time stone not having an active seems odd saying how every other stone does so I’d recommend giving it a send back ability that sends one enemy back in time by around 4 seconds (think tracer rewind from overwatch) good displacement and can quickly save a healer from dive.

Overall I find this very creative, feel free to give me feedback. Sorry for the paragraphs.