I guess I can try to talk about some of the other ones:
Mega Diancie is nifty. It's Speed isn't ideal (110 is great but it'd really like to be able to outspeed Mega Ray/Mence/Jolly Ekiller), but it has very good offensive capability, boasting 160/160 mixed offenses with STABs on Rock and Fairy. Sets will probably look like some variation of this:
Diancie @ Diancite
Naive Nature
4 Def / 252 SAtk / 252 Spe
-Moonblast
-Earth Power
-Diamond Storm
-Stealth Rock / Heal Bell / Calm Mind
Moonblast and Diamond Storm are obvious STABs. Earth Power 2HKOes Primal Groudon and takes care of most other Fairy resists. The 4th moveslot is utility: Calm Mind can potentially enable a sweep, Stealth Rock is cool on a Magic Bouncer (you can set up SR against a lead like support Dialga or Deoxys-S while they can't set up theirs, giving you momentum, free Rocks, and a free Mega Evo), and Heal Bell lets you act as an offensive cleric which is useful on a lot of teams.
This has a lot of notable advantages over Xerneas: the deer would certainly kill to get Earth Power or Diamond Storm (which makes Ho-Oh a non issue), its faster (outspeeding Palkia is sweet), and SR gives it utility Xerneas can't provide. Very dangerous Pokemon overall, even if you're busy drooling over Ray/Mence don't underestimate this princess.
Mega Metagross also probably wishes it were faster, but its still a dangerous Pokemon. 145 Attack with Tough Claws and access to boosting moves in Agility and Hone Claws make it no slouch offensively, and it is quite bulky with 80/150/110 defenses. It can also act as a solid check to all Xerneas sets, OHKOing with Meteor Mash or revenging GeoXern with Bullet Punch. It was a wide movepool to use with Tough Claws, with both STABs benefiting and lots of coverage options in Earthquake, Hammer Arm, Elemental Punches, and Grass Knot, as well as Stealth Rock to give it some added utility. Unfortunately, it's relatively low base power STABs hinder its sweeping potential even with Tough Claws, and Primal Groudon is obviously a huge pain in the butt for it. Being slower than MegaRay/Mence/Jolly Ekiller sucks pretty hard too. I don't see it getting too much usage to be honest, but its not terrible on teams that need a fast Fairy check/possible SR user that can take some hits. It also has a strong and fast Explosion for when its cornered, which might actually come in handy sometimes...
I tried Mega Beedrill today, it's a fun mon but it's not that good. It's super frail, not overly strong even with Adaptability since its STABs aren't hard to wall and it lacks really high BP moves, and usually has to waste a moveslot on Protect since its base form has no bulk and 75 base Speed. Also Primal Don/Mega Ray/Mega Mence all eat it alive which is just lulzy. I tried it on a double U-turn core with Genesect, but Genesect is pretty much just 100% better and did 90% of the work itself lol. Anyway this is the set I used:
Beedrill @ Beedrillite
Jolly Nature
252 Atk / 4 Def / 252 Spe
-U-turn
-Poison Jab
-Protect
-Toxic Spikes
Use Protect to ensure you Mega Evolve without getting killed and then pretty much just spam U-turn. Poison Jab is for Xerneas and whatever you can get a neutral hit on. If Beedrill is going to die or might force a switch, feel free to throw down some Toxic Spikes.
This Pokemon also gets Swords Dance and Drill Run/Knock Off for good coverage options, which you may be able to sweep with if you have a Wobbuffet and some luck. I wouldn't use Mega Beedrill on a serious Ubers team though.
Aside from the Dragons, none of the other Megas seem too useful. Mega Camerupt is probably the worst one by sheer virtue of Primal Groudon existing lol.
e: Actually I lied there was one more that looks sort of interesting:
Mega Sceptile is kind of useful simply because it's fast as balls (fastest Dragon in the game) and packs an acceptable punch. It's very frail and tough to switch in, but its STABs can lay the hurt on the Primals/Mega Dragons, and it outspeeds quite a bit of the metagame. In fact, it's so fast that you can run a nature that boosts Atk/SAtk and still outspeed Shaymin-S. A possible set might be this:
Sceptile @ Sceptilite
Rash Nature
4 Atk / 252 SAtk / 252 Spe
-Leaf Storm
-Dragon Pulse
-Focus Blast / Earthquake
-Rock Slide
Basicially it's a simple hit-and-run attacker. Leaf Storm destroys Kyogre and does huge damage to Primal Groudon (doesn't OHKO due to Fire-typing but it does a ton). Dragon Pulse kills other Dragons (of which there are many) and has solid neutral coverage. Focus Blast covers Dialga and Ferrothorn while Earthquake hits Klefki and Heatran. Rock Slide is for Ho-Oh.
Even with Rash nature, Mega Sceptile doesn't have a lot of damage output unless it is hitting super effectively, so it probably will not be very common at all, but it can at least perform well against those Groudon/Kyogre/Rayquaza/Salamence teams that will inevitably pop up at the start of ORAS. :p
