Here's a big question: are the first two moves always linked or are they linked only if you select the first of the two moves? Because the latter is more flexible than the former.
I don't think this is going to be as offensively oriented as people are saying. Chansey gets perfect Wish passing, Unaware Clefable gets Cosmic Boiled, in general stall can always link together healing with activity (Meaning it can't get locked into a situation of having to heal every turn and thus not get in productive moves), etc.
I really think priority has to either be the slowest priority number gets applied to the whole chain or the Pokemon uses both moves in one turn but not back to back. That is, there's no reason you can't Bullet Punch as your first action before the enemy moves, they use their two back-to-back 0 priority moves, and then you use your other move that's reduced priority.
Just flat out changing priority to always be zero or to be the first thing's priority alone is just demented, giving us stuff like Scarfed Infernape sets Stealth Rock and then Roars you out before you can move, or non-Scarfed Infernape Mach Punches you and then Roars you out at +1 priority. Or Mach Punch followed by instant Focus Punch, screw you here's 190 BP of pain at +1 priority. (Who cares about V-Create?)
I find very interesting a lot of the Volt Switch and U-Turn based shenanigans. (Victini V-Creates and then immediately U-Turns out, avoiding the disadvantage entirely) Suicide leads are also interesting -set Stealth Rock and Explode in one action.
Also note stuff like: Shuckle uses Power Trick and then immediately smacks you with Stone Edge. Then it can just switch out and do it again later. Naturally Stealth Webs is probably going to be a more popular use of Shuckle, but hey. (Though I'd prefer using Galvantula with Sticky Web+Volt Switch, Scarfed, myself)
Also note: Encore+Disable.
This is a really interesting meta. I really want to see it. And then see it win OMotM. This is amazing.
Of course it's not my decision, but I think the first two moves should always be linked no matter which one you select to use in battle. That's how it is in the PMD mechanics (not that those mechanics matter much here anyway) and it would also prevent this meta from getting too complicated.
At first I thought defensive sets would be practically useless considering all the offensive possibilities, but I see your point. It's not just sweepers and wallbreakers that get insane buffs. Every kind of play style gets new toys and strategies to play with so it's going to be much more cut-throat and intense, but not necessarily all offense all the time, just as you pointed out.
I agree with you on the priority thing, but I'm leaning in favor of the lowest priority number being applied. If we allowed both moves to be used at different times in the turn depending on their priority, it would open up a lot more variety, but a lot more complexity as well. It wouldn't make the meta awful to play in, but things could get a little aggravating, like Fake Out + Roar shenanigans. Either way, the meta will be very interesting, but the issue of how priority is applied is a big one.
When I saw that Encore + Disable combo it really had me nervous at first thinking it might be too much, but then I did a /ds on the simulator and found that these are the only Pokemon capable of learning both those moves:
It might end up being a very common way to run Alakazam in this metagame, but nothing else besides Alakazam is fast enough to make it viable. No pranksters either *breathes a sigh of relief*.
(I'm basically a PMD superfan btw, so I have no clue how no one has thought of this before. This meta is genius.)