The problem that I have with "Momentum" is that the concept is far too broad. Momentum isn't nearly as precise of an element of the game as "Scouting" or "Support". It takes form in many different ways, with every good pokemon in the game being able to effectively gain momentum, and every non-broken pokemon in the game being able to lose its momentum. Weather teams gain momentum by getting their respective weather up, so it would have to counter that. Stat-up mons gain momentum by setting up a DD, SD, etc, so it would have to be able to deal with that. Walls gain momentum by hard-countering pokemon, so it couldn't be dealt with like that, and so on. Really the only aspect of momentum that could be materialized into any one pokemon is using scouting moves like U-turn, Volt Change, or Baton Pass, but then the concept becomes "Ultimate Scout" (though I do think that would be interesting in a Team Preview metagame). Really I don't see how any pokemon could ever hope to always or even consistently shift the momentum of a match, since momentum takes so many forms. But these are just my concerns, and I would love to be proven wrong as I agree that momentum is a significant part of the game and that it deserves some insight.