Forecast:
Kyurem, Vanniluxe, and Seismitoad are expected to make water see a lot more play. Expect showers.
New fire-weak gimmick decks: Cobalion tank and Durant milling. If either of these turn out viable, it'll be a good day for fire. (not likely, though)
Basics now have their own Rainbow energy, retreat-dropping stadium, and permanent Defender. Expect evolved attackers to see less play.
Mewtwo EX is expected to become the new A-list deck. He was powerful enough before Gardevoir was revealed.
Archeon locks will be attempted. If this works out, the metagame will take a drastic shift.
Fliptini is a complete wildcard at this point.
Leavanny: coming soon to a bench near you.
A new Cincinno that probably won't be used at tournaments, but will cause lots of rage at smaller events.
Two new search trainers for times when 4 Communications just isn't enough.
A supporter that lets you grab 3 energy. Emboar's gonna love this one.
Hydreigon and Chandallure were popular enough in Japan to get shiny reprints in PD/HB. Best to keep an eye on them.
Those are most of the cards expected to affect the metagame in the next two sets. Whether or not Reshiboar will still be viable in a few months, much less a few years, is up to you.