Hi, nice team. First of all, I would advise against using Choice Scarf Terrakion over Choice Band Terrakion. First of all, it doesn't actually help you against anything. Volcarona is already handled by Heatran, while Salamence can be revenge killed by Scizor. Furthermore, Dragonite quite honestly has incredible difficulty setting up. Rotom-W can Volt Switch out, Scizor can Bullet Punch, Heatran can Roar it out, Celebi can status it, Terrakion can Stone Edge it, and Latias can Dragon Pulse. The only situation where Dragonite could possibly to set up is DD Lum Dragonite vs Celebi. I do, however, suggest swapping Terrakion to a Double Booster
variant. Currently, your team has difficulty against stall, as the only Pokemon on your team that can stallbreak is Latias, who gets shut down hard by Tyranitar, a common sight on stall teams, and Ferrothorn or Jirachi for Rain stall. Swords Dance Terrakion gives you another valuable asset for dealing with these types of teams. Rock Polish can be used against more offensively oriented teams, namely Sun teams.
You should consider a more defensive 252 HP / 4 Def / 252 SpD
spread on Heatran. The extra Speed isn't particularly helpful, but that extra Special Defense allows you to take numerous more hits, be them Draco Meteors, Bug Buzzes, Blizzards, or whatever other common move Heatran is required to take. It would be a shame if Heatran succumbed to the onslaught of Dragons because of a lack of recovery, wouldn't it?
Finally, Life Orb Starmie can rampage through your team if Celebi or Latias have taken any prior damage. A quick fix for this is to swap Rotom-W to a Choice Scarf
variant. This also allows Rotom-W to outspeed and KO the likes of Choice Specs Tornadus before it can even move, which is always helpful. Since you're not using Will-o-Wisp anyways, the ability to change moves isn't as much of a necessity. Rather than Hidden Power Grass and Pain Split, I suggest going with Thunderbolt and Trick instead. You already have Celebi to come in on the likes of Gastrodon all day long, especially with Natural Cure, so no problem there. SubDD Gyarados can just Sub up, every time you Volt Switch, KO your switch-in, and rinse and repeat until either Rotom-W is the last Pokes standing, or it is powerful enough to power through Rotom-W. Thunderbolt should patch up this problem. Trick allows you to cripple annoying Pokemon such as Blissey and Gastrodon.