Maybe it's for Dormammu.
Much like free lockdown (space) and a free damage boost (power), having your options choked by the amount of meter (don't) have is definitely something you see at high levels of play and something that's hard to say no to.
I just imagined Dormammu with the time stone too, the amount of stuff he could throw out there is probably biblical.
I don't see why Dormammu would want the Time Stone. The teleport dash is basically his default teleport. You should generally pick a stone based on its normal ability, not just its X-Factor.
If rumors about the Soul Gem are true, I think that will be the one Dormammu chooses. He can easily run away while building meter to resurrect his partner, and the passive heal whenever you hurt your opponent will make him even more oppressive.
Reality Gem seems superfluous on him, and he will crush anyone who crutches on it by default. Time Gem is superfluous. Mind Gem... Dormammu isn't going to get into range for a command grab. ROFL. Space could be okay, since you could potentially mess with their attempts to get in and move them into Flame Carpet.
Personally, Power Stone is looking to be the one that I would pick with Dormammu. It fills an actual gap of his, which is the ability to push people off of him once they get in. Plus he can combo off of a full screen wall bounce, so the synergy is strong. The Surge ability negates pushblock, which means Dormammu can really mess people up with Stalking Flare mix-ups, and of course Jedah will have a field day with no pushblock plus endless instant overheads.
So yeah, Power Stone for sure, Soul Gem is promising, and Space Gem is interesting. Unless Reality ends up being so OP that it is better than Dormammu's default tools, I don't think it is an appealing choice.
Assuming he plays like he does in Marvel. My hunch is that he mostly will, since Dr. Strange is intact. The same characters that struggle against Dormammu struggle against Strange. If one is in and the devs think he is balanced, the other probably will be. I think they might change up Dark Spells and nerf Flame Carpet, but the basic style will be the same.