I am hoping for some very unique types. There has never been a Grass/Fire or Fire/Fairy. If they do that, they could make the Duck a Water/Bug (only three in existence) to give each an advantage and resistant against the Starter they are weak against.
Doubt it will happen, but that would be cool. If it someone does, they can make the Duck evolve into more like a Hummingbird Hawk Moth, as that is really the only bird insect in existence.
But like you said, Duck will likely be another Ducklet as Water/Flying or the next Water/Dark after Greninja. Fuecoco will probably be Fire/Ground like since the only other Croc Poke, Krookodile, is Ground. And Grass will likely be Poison or Fighting since they seem to be pretty common.
Even making them all Dual Normal would even be against the grain. No real advantage except extra Stab Move, but still.