We're going to see a big difference in pokemon usage
The CP cut off tiers already drastically changed the way we were going to battle in PvP ontop of it just by concept being more TDO oriented than our common DPS heavy meta, but now the additional charge move slot will more than revolutionize the way we play.
The big one I was thinking of was Mewtwo versus tyrantiar. Normally this would be a very hit or miss match up, mewtwo caries only shadow ball, tyranitar with bite/crunch, game of for mewtwo, and you'd probably need to carry a machamp to just wall the tyranitar or the mewtwo's got focus blast which (likely from mewtwo raid experience) one/or nearly shots the tyrantiar but is very vulnerable to a lot of other legendaries, including a shadow ball mewtwo. Now combine these two sets, the mewtwo caries shadow ball and Focus blast and the tyrantiar caries bite/crunch and maybe fireblast. Similar satiations happen with Kyogre with blizzard, Ho-oh with solar beam when it comes to their coutners, but this match up illustrates this most clearly in my oppinion. This could make a lot of pokemon significantly more tricky to deal with, as they wouldn't need to depend on another pokemon (or half of the rest of the battle party) being meant to defensively counter an oppoent.
That and it gave usage to a lot of tanky legendaries, things like cresselia, and even the community despised regi's look like they could be serious forces to be reckoned with because for their massive defenses and low over all CP
