This might be coming a bit late, but to balance anything in this game requires a baseline to somehow quantify the changes. Usually I would take the ingame difficulty as baseline for balancing. Taking the highest or most played difficulty seems reasonable, but it will also lead to some missing balance on difficulties below.
Now V2 is a bit problematic, since there are paywalled difficulties that have a consistent playerbase, but it does not necessarily mean to represent the majority.
Recently I posted a small poll on reddit asking which difficulty the balance should revolve. The Poll (login to see results). And a majority opted for either Legend or Legend & Cata.
Now 300 people will barely represent the whole community, but still the main argumentation I saw, was that the game should be balanced around the highest non-paywalled difficulty.
Personally I use cataclysm as baseline, but I wondered where the developers or active players of this forum set their baseline.