At first I thought Soundproof would be the standard ability to avoid getting demolished by Sylveon and M-Gardevoir's Hyper Voice, however it does need to invest 60 EVs into Defence to avoid the OHKO from Psyshock from Timid M-Garde, and a much larger investment (probably not worth it) to survive Modest M-Garde's Psyshock. Furthermore, at +1, it OHKOs Mega Gardevoir with Poison Jab anyways.
+1 196 Atk Kommo-o Poison Jab vs. 16 HP / 8 Def Mega Gardevoir: 312-368 (111 - 130.9%) -- guaranteed OHKO
252 SpA Mega Gardevoir Psyshock vs. 0 HP / 60 Def Kommo-o: 246-290 (84.5 - 99.6%) -- guaranteed 2HKO
If Sylveon ever became a relevant threat again, I could see it being useful, but with that in mind I would say either Overcoat or Bulletproof would be better for now. Overcoat notably stops Tangrowth and M-Venusaur from Sleep Powdering you and blocks Spore from Amoonguss and the odd Sash Breloom. Bulletproof lets you completely fuck over Gengar (as if Gengar wasn't getting fucked over hard enough already) but sadly does not block Moonblast. Given that users of bomb/ball moves that aren't Gengar (who is already outsped and KO'd at +1) are fairly sparse in OU, I would say Overcoat's utility trumps it.
With that being said, this is the set I'd probably use if I were to use Kommo-o. I'm not sure if it's going to be OU viable or not, it really depends on what the S/M metagame ends up looking like. At the very least he'll be a solid choice in UU, although I suspect Flygon may give him competition in that regard, as he offers an arguably even better STAB with a better defensive typing and Roost, at the cost of some bulk.
Kommo-o @ Lum Berry
Jolly - Soundproof
252 Speed, 196 Attack, 60 Defence
-Outrage / Dragon Claw
-Sky Uppercut
-Poison Jab
-Dragon Dance
I don't really think I need to explain this set much, we've all seen a Dragon Dance sweeper before. The classic Outrage vs Dragon Claw conundrum still applies here. I desperately wish he got a better Fighting STAB than Sky Uppercut but... He doesn't. Here's hoping for Drain Punch from a Tutor in the next games... Poison Jab covers most Fairies, what would otherwise be his number one weakess. He somehow didn't get Iron Head so Magearna and Diancie are still gonna be pains in the ass, but if you really wanted to cover those threats you could pack Earthquake somewhere.
Yeah, I don't think he'll be relevant over Dragonite, Gyarados, M-Charizard X, M-Altaria, or even Salamence, but assuming those threats all say in OU/BL, he has the potential to be one of the most solid Dragon Dancers in UU (assuming he stays in UU himself, which given all of his flaws, I think he will). If he gets some neat toys from future games such as a better Fighting STAB, he could potentially be useful even in OU, but sadly as it stands there's just not much reason to use him over other Dragon Dancers.
Regarding Shell Smash, if he ever does get it, then obviously that's a game changer and immediately makes him OU viable. A set with Shell Smash / Outrage / Focus Blast / Poison Jab could be devastating, or even a full special set of Shell Smash / Clanging Scales / Focus Blast / Flash Cannon.