Yeah I dunno how well Mega Diancie can counter Mega Rayquaza. Ray can OHKO M-Diancie Earthquake (guaranteed) or Aqua Tail (75% chance), and Jolly / Naive outspeed M-Diancie...
I think Diancie would be best used as a non-mega Trick Room setter, it can even run Gravity and let Groudon wreck with both up. Be prepared for this! it worked wonders in the Doubles comps with Cres coz of levitate, but any Trick Room + Gravity user will work coz you can rotate to Groudon straight away and spam Precipice Blades. Diancie also gets Heal Bell to help against Dark Void and other things...
I think Uxie could be cool here as the only mon with access to Yawn. I learned the hard way in the last Rotations comp (Eevee Friendly) that it's really annoying since you either have to switch (which is not what you wanna do in rotations) or let something sleep. He also gets screens so that's always good
Jirachi is also something I wanna look into as only one of two users of Wish (the other being Latias but she won't be able to live too many hits from stuff like M-Ray / GeoXern etc. to use it as well). The plain old support set of Wish, T-Wave, Iron Head and [filler] (maybe something like Cosmic Power or Doom Desire) would be cool imo.
Toxic is also something to look out for. Since rotating =/= switching the Toxic damage continues to rack up. However cleric support will probably be a thing coz of Darkrai so maybe it won't be as potent as I'm thinking...
Choice attackers aren't something I'd recommend in Rotations (except maybe Banded Groudon with Trick Room + Gravity support) since they're too easy to exploit without being able to switch out and reset what move you're locked into..
People keep mentioning 'Primal' Groudon and Kyogre. These guys won't be as good as their regular formes in this competition due to the way their Primal abilities work in rotations. As Jibaku already explained, they don't activate when rotating in and they actually deactivate (unlike normal weather) when rotated out, so if you lead with them and get their Primal Weather activated, you'll only be able to make use of it until you rotate out and it will go away. Due to this, I seriously think Choice Band (or Life Orb / Earth Plate if you want flexibility) Groudon will be the new thing over its Primal as you can spam Precipice Blades with Trick Room and Gravity set up...