I don't think that Tavern Brawls take up -that- much of Blizzard's time. For this first Tavern Brawl, they pulled directly from the Blackrock Mountain Adventure. Tomorrow, we'll see what the next one is, but if I had to make a guess, I'd say it'd be something like "Now that you've seen how Nefarian and Ragnaros work, make your own decks with the heroes / their cards!".
Point being, they have a bunch of work that they've already put into their Adventure modes to pull from, and Tavern Brawl allows them to reuse that work in a PvP environment.