Now, one line of thought would be that all crossover pairings are rare, so there's no point in considering them for this month when we'll have a crossover challenge soon enough. But I've found that in at least one of the more populated crossovers I've seen, there is one crossed couple who pretty much dominates. So any other couple that was one from each fandom would, imo, qualify as a rare pair above and beyond the rarity of the crossover.
What do you think? Is any cross fandom pairing rare, or are there some that dominate enough to not seem rare to you in your favorites? What crossover rare pair would you like to write or see?