Hey Yilx,
This team looks really solid already so there aren't many things you should change. On your Heatran/Celebi doubt, I would recommend using Heatran, as you can already deal with Rain teams fine with Latias and Rotom, while Volcarona is almost an instant lose if you aren't using Heatran. Speaking about Heatran, you should try either a Timid Nature on it or Roar as with them you can either Toxic Substitute Gyarados (or phaze it) to prevent it from setting up, as it can sometimes beat Rotom-W thanks to Substitute, especially if it switchs in when rain isn't up. Hope I helped!
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