With SC2 accounted for (other than post-launch support, whatever form that takes, but it's not like that requires substantial engine work, tools development, and all the other stuff that keeps the RTS team busy), the next Blizzard RTS is likely already in the engineering and proof-of-concept phase. If memory serves, SC2 was already in that phase not long after The Frozen Throne was released, although it was years before production was in full swing and they didn't have anything public-facing to show until 2007. (Browder came aboard to take the lead in 2005, I think.) But as with D3, when SC2 was first shown you could tell that a ton of the core engine work was already finished in much the form we still recognize today.
I severely doubt WC4 will be set for announcement this year, but Blizzard now has a freer hand than ever to work on it, and RTS engineering staff have probably been moved off SC2/Heroes for a long time now anyway, apart from a skeleton crew for refinements and optimizations. LOTV campaign has been in the can for a long time now, and we don't know what their RTS campaign scenario designers have been working on other than the co-op missions, epilogue campaigns, and maybe the mysterious post-launch stuff we haven't heard about yet.
All speculation on my part, of course; it's hard to tell what goes on internally. For the longest time it looked like a ton of SC2 staff had been moved to Heroes, but then they came out and said their team has never been bigger, and they are in fact still hiring for it.