With the new Kami trio formes revealed, I'll guess what roles they may be playing in the metagame. These might have been said before, but I might elaborate on those.
Tornadus: with it specializing in high defenses and Speed, I'm guessing that it'll be more of a supporter than before, utilizing Prankster to set up Rain Dance for non-Drizzle Rain teams, and tanking a hit after with its high defenses, then U-turning out or attacking when necessary. Of course, Stealth Rock is still going to be a pain, and I hope GameFreak will make Roost a move tutor move that the Kami trio will be able to learn.
Thundurus: it's still going to tear things to shreds in Ubers with its high Special Attack. In other words, that new forme may perform the same role Thundurus' current forme does right now (pre-Uber and post-Uber).
Landorus: I dunno, but I think its new forme may perform the same role its original forme currently does in OU.