I had a thought about a very special hypothetical pokemon move. One that, when you use it, allows you to pick two of your other moves in a sequence of your choice to use in one turn cycle. The move itself would be priority, but when the two other moves are used depends on the pokemon’s speed or that move’s own priority. What would you say the implications of such a move would be?

Well, if I’m understanding you correctly, you can use two moves in one turn.  You have to give up a moveslot to do it, which is a big sacrifice, fair enough, but in return you’re getting probably about 50% more attack power (being conservative because your second attack might be made without STAB, or be resisted or something).  People happily give up the ability to switch attacks for that kind of power (by using a Choice Band or Choice Specs).  That’s also without getting into the kind of combos you can pull.  You can use Dragon Dance and Outrage in a single turn, potentially giving your opponent no chance to respond if you were already faster than whatever they had out.  Ghost-types can paralyse you with Thunder Wave and follow up with Hex.  Dream Eater is finally viable against human players for the first time in Pokémon’s history.  Basically, you get one attack, which would otherwise be a perfectly good use of your turn, and then you get a second attack with some kind of bonus which would normally require careful set-up or big risks.  It also magnifies the number of possibilities your opponent has to be able to prepare for on any given turn.

I think this would be a very fun signature move on a specific Pokémon, something with lacklustre stats, but an interesting movepool with lots of neat support techniques (something like Spinda, maybe?).  The idea of making it widely available sets off a series of very loud and insistent alarm bells in my head, though.