1) Has the metagame become more offensive or more stallish ?
Neither, really. BW2 Granted us both about the same amount of defensive and offensive mons. Lugia, Ho-Oh, Lati@s, Landorus-T, and hey, even Genesect can be offensive and defensive (well you probably aren't going to use Gene to tank hits but it checks stuff pretty well and is one of the better ways to annoy the living daylights out of Mewtwo). Spikes still remains supreme imo and it's probably not changing for a while - unless a massive cult of antispikes ladderers come by of course xD
2)What are the rising underrated threats that weather can abuse in the current metagame ?
I'm not sure what exactly this is asking - do you mean the mon abusing weather? Anyways, I used defensive Landorus-T early on and it's great. The 4x Ice weakness is rather annoying to deal with though as a random Tyranitar could probably just Ice Beam is to death but otherwise it's a solid Ground physical wall choice for rain teams, only really competing with Ground Arceus. But surely, this means you could use another Arceus forme?
3)What type of anti metagame pokemon do you predict ?
Because the metagame doesn't particularly favor certain playstyles this can be rather hard to pinpoint. I'd say approaching the anti metagame is best done by devising certain sets and spreads on some existing Pokemon moreso than finding a new Pokemon. Due to the versatility of them Pokemon in the metagame, it is very difficult to cover up everything each of them can do. Something like Whirlwind Ho-Oh can completely dismantle teams that rely on Giratina to wall the regular Ho-Oh. Thunder Wave Kyogre is also a nasty surprise to the common Latias switch in.
4)Is dedicated Rain / sun offense still as good as in BW ?
Yes, although dedicated sun offense was always borderline in terms of being good. Dedicated Sun offense has the potential to output crazy amounts of damage but is more weather reliant than rain (IMO) and tends to dislike Stealth Rock more. On the rainy side of things, imo one of the biggest problems rain offense had back in DPPt was Lati@s, and with their return in BW2 they are still something to keep in mind, especially if they have Thunder. However, BW2 overall should have an easier time dealing with them, with Arceus and Genesect available. But really, both offensive playstyles should remain as viable as they were in BW