Personally, I think a Fire/Dragon cover Legendary for Sun and an Ice/Fairy cover Legendary for Moon would be cool. That way, Sun's Legendary has the Fire type to represent the daytime's heat and the Dragon type to represent the Sun's intensity, while Moon's Legendary has the Ice type to represent the nighttime's coolness and the Fairy type to represent the Moon's serenity. Plus, Fire beats Ice and Fairy beats Dragon, so each Legendary has one type that beats their counterpart and one type that gets beaten by their counterpart. It both makes sense and is functional, and that makes it interesting, in my opinion, even if Reshiram is already a Dragon/Fire-type cover Legendary. Just my two cents, of course.
Also, I apologize if I screwed up the capitalization of "sun" and "moon". For me, it's a really confusing subject, but I've always been taught to capitalize them only when they're preceded by "the", so that's what I did. I believe there are multiple acceptable ways to capitalize "sun" and "moon", however.
Mewtwo looks amazing in Pokkén Tournament! I've been waiting forever for N60Sean to upload the Mewtwins, so I'm really glad that he uploaded Mewtwo. His videos are really good. If you haven't checked them out yet, I highly recommend them. Anyhow, Mewtwo's moveset looks very cool. Lots of psychic blades and energy attacks. In particular, I think Telekinesis and Psycho Cut are animated quite well, and Teleport and Drain Punch seem to have interesting applications. I'm not really sure what Barrier does, though. Given Barrier's effect in the mainstream Pokémon games, I imagine that it reduces the amount of damage Mewtwo takes per blow, but I don't know for sure. Also, Barrier looks to be Mewtwo's version of Shadow Mewtwo's Reflect, in that both appear to be gateway moves. However, I think Reflect is kinda like a midair shield, whereas Barrier seems to be a stat-enhancing move. I can't wait until N60Sean uploads Shadow Mewtwo's moveset so that we can compare him to Mewtwo! Oh, and, in my opinion, Low Stance Strong Attack, Psystrike: Final Blow, and (especially) the Elemental Punch chain are Mewtwo's coolest-looking moves.
Finally, since my favorite Pokémon type is Poison, I would love to see a Poison-type Legendary Pokémon! One idea that I'm partial to is a Poison/Ground dual-type, mummy-esque Pokémon based on Pestilence. It would especially cool if three other Legendary Pokémon based on War, Famine, and Death formed a quartet with it, creating the Four Horsemen of the Apocalypse, kind of like how the Swords of Justice are based on The Three Musketeers. Maybe the Four Horsemen could even serve as a counterpart quartet to the Swords of Justice. That'd be pretty neat...