Just an update: because they appear to be a common request, I've cloned the Ditto for future trading. If you want one, it's available. I know of a few Pokémon that are not in B/W at all, so how about either an Azurill/Marill (the latter preferred as it's friendship-based and I'll catch Azurill to evolve in W2 in any case) or a Mareep?
Really, anything exclusive to B2/W2 is good (except Elekid, someone traded me one already), and any gaps will be covered someday by imports from other games; such trading limits the amount of importing I have to do. :)