Hi False Sense
This is a very solid and unique team you have here. I really like your use of some Pokemon that aren't too common nowadays such as Hydreigon and Offensive Celebi. As previously mentioned, Quiver Dance Volcarona
is a major threat to your team. After a Quiver Dance, he can really pack a punch to your team with Fire Blast and Bug Buzz. Making some changes to your team to compensate for this Volcarona weakness would open you up to more weaknesses to Fighting-type. The best bet I think is trying a unique set in Bulky Support Gyarados
with a moveset of Thunder Wave / Taunt / Dragon Tail / Waterfall
over Rotom-W. Gyarados resists both of Volcarona's STABs while being a solid check to opposing Fighting due to both resistance and Intimidate. Thunder Wave helps slow down your opponents which is very useful for Hydreigon, Celebi, and Lucario, all of which are not the fastest of attackers. With paralysis support they can now outspeed many threats they could previously not outspeed. Taunt prevents your opponent from setting up hazards, statusing Gyarados, or trying to set up on him. Dragon Tail is a nice move to phaze out the opponent while dealing damage and forces the opponent to take extra Stealth Rock damage. Lastly, Waterfall is a solid STAB attack that deals solid damage even uninvested due to his high base 125 Attack. Gyarados makes a decent check to sun teams, and together him and Celebi can take on most rain teams as there is no more Tornadus-T.
Now for some smaller changes. I think Roost
should be the primary option in the last moveslot for Hydreigon. Hydreigon already has great coverage with its 3 attacks and Roost left it heal off Life Orb recoil, giving you increased survivabilty. I'm also going with Electrolyte's suggestion of Surf over HP Fire on Latios
. Surf offers generally better coverage than does HP Fire, lets Latios hit things like Heatran and you won't have to sacrifice a Speed IV which is crucial as it would cause you to lose a potential speed tie against opposing Scarfed Latios.
Hope I helped and good luck with your team!