Could just be because Water and Fighting are my two favorite types, but I think about this from time to time: We're six generations old now, and yet the only garden-variety Water/Fighting we've got (Keldeo isn't "garden-variety", before anyone even mentions it) is Poliwrath. This could probably be said about [your favorite type combination that hasn't been done much here] as well, but I can't help but think we're overdue for another one. At the very least, it'd be cool to get a Water/Fighting starter some time down the road, if nothing else.
So what do you think? Is Poliwrath enough, or would you be okay with an alternative take on Water/Fighting? Could a new Water/Fighting do something that Poliwrath and Keldeo don't already do? Given the choice, what would you want a new Water/Fighting-type to be based on?