[Deck Knight]

My take on this is that we want to be countered by things you'd see on an opposing Sun team, and threaten things you'd see on other teams. To this end, I think we should threaten:

Scizor
Jirachi
Ferrothorn
Latias
Latios
Toxicroak
Politoed
Tentacruel
Gastrodon
Rotom-W

And be threatened by:
Venusaur (check)
Heatran
Ninetales
Volcarona
Infernape
Rotom-H
Hydreigon

To do this I would support Fire coverage, simply because Rain teams are not going to use Fire coverage CAP 5 to take out opposing Ferrothorn. While there are arguments to be made against CAP5 threatening itself, the brute fact is that Fire/Grass/Dark coverage does fairly well against non-sun Pokemon and is mediocre against sun-loving Pokemon, and there was always the issue of running HP Fire in Sun to take advantage of the boosted STAB and the ability to wipe out Scizor. In this case why not just go indy and make it official. Fire coverage hits the threats we want it to hit, encourages sun, and doesn't hit things we don't want CAP5 to threaten.

Salamence and Dragonite are kinda neutral since they get powerful physical and water special attacks with the stats to back them up, at least in Rain, whereas Hydreigon really prefers Fire Blast over Aqua Tail or Surf.

The simple fact is you can't counter all the Dragons, so we should counter the dragons that thrive best in the opposing playstyle to the best of our ability, and encourage Dragons that can thrive in ours. Using out Dark STAB to counter the Latis accomplishes this while leaving Dragons in general to other means. Dragons that choose to run Fire Blast will not generally run on Rain teams.