Druddigon
Looks pretty good, but I would change the EVs to 252 HP / 252 Def / 4 SDef and an Impish Nature, and change Aqua Tail to Dragon Claw for a more reliable STAB.
Latias
Only thing I'm gonna change is the spread. 252 HP / 4 Def / 252 Spd so you can take advantage of its above average speed and set up fast.
Dragonite
I can kind see what you're going for, but it won't really work that well tbh. Try a band set to give your team some raw power right off the bat.
Dragonite @ Choice Band
Trait: Multiscale
EVs: 252 Atk / 252 Spd / 4 HP
Adamant Nature
- Outrage
- ExtremeSpeed
- Fire Punch
- Earthquake
Latios
I recommend a Scarf set so it can function is the team's revenge killer. It also outspeeds +2 Cloyster, which is very helpful for this team.
Latios @ Choice Scarf
Trait: Levitate
EVs: 252 SAtk / 4 HP / 252 Spd
Timid Nature
- Draco Meteor
- Psyshock
- Surf
- Trick
Haxorus
Good, but change the spread to max speed and attack, and change Brick Break to Superpower to hit harder. This is just a personal preference, but Jolly > Adamant.
Kingdra
Nice, good cleaner for the team. 252 Spd / 252 SAtk / 4 SDef is better for the spread in case you need to attempt a sweep without rain.
Importable
Druddigon @ Leftovers
Trait: Rough Skin
EVs: 252 HP / 252 Def / 4 SDef
Impish Nature
- Glare
- Dragon Tail
- Stealth Rock
- Dragon Claw
Latias @ Leftovers
Trait: Levitate
EVs: 252 Spd / 252 HP / 4 Def
Timid Nature
- Calm Mind
- Roost
- Dragon Pulse
- Hidden Power [Fire]
Dragonite @ Choice Band
Trait: Multiscale
EVs: 252 Spd / 252 Atk / 4 HP
Adamant Nature
- Outrage
- ExtremeSpeed
- Fire Punch
- Earthquake
Latios @ Choice Scarf
Trait: Levitate
EVs: 252 SAtk / 252 Spd / 4 HP
Timid Nature
- Draco Meteor
- Psyshock
- Surf
- Trick
Haxorus @ Lum Berry
Trait: Mold Breaker
EVs: 252 Atk / 252 Spd / 4 HP
Jolly Nature
- Dragon Dance
- Outrage
- Earthquake
- Superpower
Kingdra @ Life Orb
Trait: Swift Swim
EVs: 252 Spd / 252 SAtk / 4 SDef
Modest Nature
- Rain Dance
- Draco Meteor
- Hydro Pump
- Hidden Power [Electric]
Hope this helps and good luck! ;)