I think it is going to be interesting to see how the competitive scene changes with the new games. It'll be the first time I've really been involved in that side of the game when a new game comes out. We saw it a little bit when ORAS came out and introduced a bunch of new Megas, but a bunch of new Pokemon should produce a more significant shift. I've seen most of the new Pokemon, but I don't know any of their abilities (done this on purpose, please no spoiling), so I'm not sure how competitive it seems they are going to be, but it should be fun.
Not sure how many new Pokemon have been announced at this point, but apparently they are doing another info dump next week. I really wish they would stop. I'm trying to stay away from spoilers but it's not working very well. I think I've managed to keep some of the new ones a secret from myself, but I'm not sure. Just seems like they've released a lot of information already at this point.
Also, I'm really burnt out on the legendary SRing. Anyone know how many you can backlog with the girl that gives them to you? I think I have almost 10 in one of my games at this point.
I'm also very interested to see how it will change.
Besides the new Pokémon, we must not forget that new moves will be introduced, obviously, and older Pokémon will also get access to those. Same could happen with the new abilities. Some already existing moves may even be added to existing Pokémon movepools (again, same with abilities, but it would surprise me a bit more - the moves can be for potential new megas).
Will be interesting to see if terrains can become competitively viable too (ie. if they become accessible and easy to set up like weather) - which could be a huge game changer.
You have grass terrain which is insane for tanky stall teams because it's like leftovers (though for both party).
Electric terrain would **** up Smeargle in VGC because RIP dark void (same with the likes of sleep powder/spore and of course dark void in single).
But most importantly... Misty terrain. This is like safeguard, basically, and it halves damage done by dragon type moves, which could be huge against an opponent whose only unresisted move is a dragon type move.
Just wish they would differentiate electric and misty a bit more, though.
Other potential gamechangers ...
[spoil]are the speculated synchro evolution and/or the "special/break/ultimate/etc." moves. But I've already speculated myself about those a lot.[/spoil]