If this bulky offense trend continues Suicune will be OU in a heartbeat. He counters so many offensive threats like Talonflame, Blaziken (both mega and LO variants), any Genesect that's not using Thunderbolt (and even then it needs a +SpA download boost to do any real damage, which Gene doesn't get if you have at least 1 CM), Excadrill, Lando-T, and of course can set up on plenty of stall mons as well.
Last gen, Keldeo and Politoed were better choices for waters. Politoed gave you rain, while Keldeo could abuse that rain or just abuse his scarf set which was basically unstoppable by HO teams. This gen, there's no perma rain so defensive Politoed loses most of his utility, and if you want a bulky water you don't really have a lot of other choices. Keldeo is still decent, but new offensive threats like Talonflame and mega Pinsir hard counter him, while defensive teams have fairies, who resist secret sword, generally have high SpD for surf, and can hit back with super effective moves like air slash or moonblast. Vaporeon and Tentacruel are still okay options, but without the permanent rain that they benefitted from so heavily last gen they are both inferior to Suicune when it comes to taking hits and dishing them out.
I've made like 4 posts so far on Suicune. He's really, really, really good. Especially right now since no one is using any electrics other than the Rotoms (who generally only have volt switch, which means if you need to you can just take the hit and start boosting, then be +2 by they time they come back in to hit you again).
As far as which pokes will fall? I believe the aforementioned Vaporeon will for sure. Tentacruel still has good typing and knock off so he can be a useful pivot in OU, but his usage is going to drop.
Heliolisk is a straight shot for UU with a little bit of OU viability. The Rotoms and Thundurus are better as general electrics, and plenty of other pokes do the glass cannon thing better with higher special attack and speed (sup Gengar), but his unique speed and ability make him far from useless.