I really can't see Perth's opening home game being anything but at Optus, huge crowd, and no double-header. I also can't see it being a blockbuster game (ie: vs any 'rivals', or vs any team with big fan base in Perth already). You save those for later when the initial hype dies down. That rules out Souths, Manly, Warriors, Brisbane. Maybe even Dolphins by then. Perth vs Melbourne could be viewed as a 'rivals' game, so rule them out.
A Dragons, Parra, Gold Coast, Cronulla, ect, make the most sense to me. Dogs, even. If they're still going okay, they'll have a strong contingent probably flying across for it. Adds to the atmosphere.
First 'Sydney home game' for Perth, likewise, should be against a team not labelled a 'rival' for the same reasons. It should be a packed house regardless of who the opponent is, so throw in a lesser supported team for Perth to play against. Probably a Sydney team for that one. Roosters, Wests maybe.