Your variant of Gardevoir is interesting, but ultimately not very good. This is due to Gardevoir's inherent frailty and sub-par speed stat, meaning it results in an almost useless mon with only a single purpose: trolling unsuspecting idiots. A competent player will easily put this thing down in the 3 turns it requires for set up (Mean Look, Encore, Disable). That said, a defensive Fairy type would certainly be a boon to this team.
Clefable @ Leftovers
Ability: Magic Guard
EVs: 252 HP / 252 Def / 4 SpD
- Stealth Rock
- Thunder Wave/Heal Bell
Clefable allows for a decent Fairy wall to exist on your team. Moonblast and Softboiled are pretty self-explanatory. Stealth Rock allows you to set up hazards on the switch, which is handy in wearing the opposition down. Finally, a Thunder Wave user may be useful due to the lack of a Scarf revenge-killer, and can essentially neuter some threats like Scarf Lele. Alternatively, Heal Bell might be used in case of status afflicting one of your offensive mons.
Your Volcarona set is decent, however the dual STAB combo is becoming less and less threatening as things like Tapu Fini, Keldeo and Heatran run around. Instead, a more offensive set would look like this:
Volcarona @ Firium Z
Ability: Flame Body
EVs: 252 SpA / 4 SpD / 252 Spe
- Quiver Dance
- Fire Blast
- Giga Drain
- Hidden Power Ground
Although Fire Blast doesn't boost up as Fiery Dance does, the raw power it provides with Firium Z makes it a worthy replacement. Giga Drain allows a form of recovery as well as hitting Water types like the aforementioned Tapu Fini as well as Manaphy. Finally, Hidden Power Ground stops you from being walled by Heatran.
Because of the new Volcarona set, you need a mon that can pressure Stealth Rock users such as Lando-T, Garchomp and Heatran.
Greninja is definitely the right call here, although having an Expert Belt is generally inferior to a Life Orb set. That said, it can be run, just a little less efficiently than the LO set.
Greninja @ Life Orb
EVs: 4 Def / 252 SpA / 252 Spe
- Hydro Pump
- Dark Pulse
- Ice Beam
Because Greninja threatens a huge portion of OU, many are forced to switch against it. You can punish this by layering Spikes down, which allow you to wear down the enemy team. The most common Defog users in the tier (Mew, Latios and Zapdos) are all immensely afraid of Greninja due to its power and coverage, essentially allowing you freedom to keep your hazards up. Hydro Pump functions as a catch-all offensive move, whilst Dark Pulse hits Mew. Ice Beam hits Tangrowth, Zapdos and Zygarde.
Lucario is unfortunately useless in the current OU metagame, as is Mega Garchomp. Instead, I replaced them with Mega Scizor and Zygarde.
Scizor @ Scizorite
Ability: Light Metal
EVs: 248 HP / 64 Atk / 40 Def / 100 SpD / 56 Spe
- Swords Dance
- Bullet Punch
With great synergy between Clefable, Scizor and Zygarde, you should be able to play around most opponents teams with combinations of smart switching and offensive capability. Swords Dance gives great offensive presence, Bullet Punch and Roost are self-explanatory, U-Turn conserves momentum.
Zygarde @ Leftovers
Ability: Aura Break
EVs: 240 HP / 152 SpD / 116 Spe
- Dragon Dance
- Thousand Arrows
- Extreme Speed
A more defensive Dragon that can explode on passive mons. Sub allows avoidance of status, DD gives offensive presence, Thousand Arrows hits pretty much everything and is the most spammable move, and ESpeed is just here for pesky priority users that might wrecks your Sub.
Finally, hazard control is needed in case Greninja can't threaten the Stealth Rocks users out. Rapid Spin would be nice to keep our hazards up, but to be honest our Spikes and Rocks are just nice things to have rather than win conditions.
Due to this, I chose a defensive ground immunity that would also enable hazard removal, as well as status spread to let Volcarona and Greninja run wild.
Zapdos @ Leftovers
EVs: 248 HP / 240 Def / 20 Spe
- Hidden Power Ice/Toxic
Discharge distributes paralysis, which frees up Clef to use Heal Bell and alleviate status on your own team. Roost is pretty obvious. Defog clears out Rocks which lets Volcarona have a bit more freedom. Finally, Hidden Power Ice can be chosen to nail Lando/Garchomp/Zygarde, or Toxic to wear down tougher foes.
Hope this all helps you