Gonna post my nerdy thoughts
Genesect
I feel like the meta would consist more on VoltTurning and hazard sacking w/ this thing around. It is a great clefable check w/ Choice Band however can't switch-in freely to twave or a flamethrower (which you don't see anymore, interesting). More Fire types are gonna be used, mainly looking at Heatran, also mening bulky waters like Azumarill and Keldeo now have field days in genesect meta. considering on how fast the meta is currently, scarf genesect STILL puts in work and does what it does best, fast uturnign and revenge killing. I will as a downloaded +1 atk CB STABs + Xspeed is pretty fuckin lethal.
Mega Mawile
I feel like unbanning this would just make the metagame more sucker punch weak again like it was during XY. The only bisharp checks i'm seeing in today's meta are either Keldeos or Hippos and both can get worn down pretty quickly. What I love about Mawile is it's ability to wallbreak and stallbreak with ease. What I don't like about it is for this exact reason as well, you can't switch-in vs it and it's kind of 50/50s every turn like w/ bisharp, except more powerful. It'll just make ORAS OU more offensively based than it already is. Big Dick Plays all day pretty much.
Blaziken
Lol, no
Greninja
Tbh with this coming back, things like torn-t and weavile might downgrade in useage because greninja fulfils the same rules as they both do, Hit Hard and Run. It's a great mix atker and got extremely buffed with the Gunk Shot and Low Kick tutor moves which we thought would change anything but it completely dominanted and was later deemed the mon "no switch-ins" which was a pretty big deal before it's ban.
Reshiram
This will be interesting. You never see this used in Ubers, at least I don't. It has a decent movepool and we can always use the scizor checks. I just feel like it'll just get outclassed by many other dragons in ORAS OU anyway, as it might have the same typing as a Zard-X but yet no access to Dragon Dance. It could be a decent scarfer or a wallbreaker w/ Specs or LO. I see shit like Azumaril being used a bit more than it already is w/ this running around.
Deoxys-D and Deoxys-S
I mean... It's just more Hyper Offense w/ Bisharp.... That's fun....