In terms of supporting team members, looking at Arceus-Dragon's set, I believe it will have problems with bulky Steel-types, and will also have trouble with opposing scarfed Dragon-types. Therefore, a good defensive Steel-type on the team will probably work wonders. I'm thinking either a Dialga or a Forretress will work for our next pick. Dialga, having a Dragon neutrality, can typically defeat other Dragon types, has Fire Blast to roast problematic Steel-types such as Ferrothorn, and can support the team with Stealth Rock. Forrestress can tank Dragon-type move aimed at Arceus-Dragon, use Rapid Spin to clear the field of Toxic Spikes, and has access to all hazards, making it an excellent supporting member. However, it has a 4x Fire weakness that we will have no control over (no Rain support and possible Sun, depending on the opponent's team).
Also, Poppy, I believe that this CCAT doesn't allow weather, so I don't think Rain Balance will be possible (Kyogre).