I can't imagine Wings of Liberty-only questions being the norm. Going by Blizzard game history, expansions usually supersede the original games such that nobody generally plays the original by itself except for the campaign.
And, while I'm sure Heart of the Swarm will cause a spike in activity, I can't imagine a whole lot of Heart of the Swarm-only questions (or more specifically, Heart of the Swarm versions of existing questions) that aren't some form of "How did X unit/mechanic/map change in Heart of the Swarm?"
With that in mind, any non-campaign questions we have now should likely just be updated with the new changes brought on by the new expansion. If someone comes across a question that truly is only applicable to one expansion, can't be updated, and would only cause confusion by not being tagged with the expansion, then it should be tagged with either starcraft-2-wol or starcraft-2-hots as appropriate.