Well... 'The Unremembered Empire' hasn't been released yet. Do we even have a release date for 'Warmaster'? And then there's also the Bequin trilogy to take into account. My guess is that it's going to be tough for Dan to write a third non-Heresy novel in a row.
If Dan remains ambitious with his schedule, I think you'll see this order:
1. 'The Unremembered Empire' (third quarter 2013)
2. 'Warmaster' (estimated for December 2013)
3. 'Ravenor versus Eisenhorn' Part 2 (second or third quarter 2014)
4. Heresy novel (fourth quarter 2014?)
5. Iron Snakes Space Marines Battle novel (sometime in 2015)
6. A sequel to Titanicus (or was it Double Eagle?) (sometime in 2015)
7. I couldn't guess when the theoretical Iron Snakes serialized novel would come out
... But that's just my guess. I base it on his bibliography/release schedule, which shows he is able to knock out two novels a year, but has also had to take a break (and a well-deserved one, at that - health related in one case, I think!) every couple of years. I include only his schedule since the Horus Heresy series started (for obvious reasons):
The Armour of Contempt (2006)
Horus Rising (2006)
Ravenor Rogue (2007)
Brothers of the Snake (June 2007)
Only In Death (October 2008)
Titanicus (October 2008)
Blood Pact (May 2009)
Prospero Burns (November 2009)
Salvation's Reach (October 2011)
Pariah: Ravenor Versus Eisenhorn (October 2012)
Know No Fear (March 2012)
The Warmaster (December 2013)
So, in eight years, Dan's been able to give us four (soon five) Heresy novels, four (soon five) Gaunt's Ghosts novels, two Inquisitor novels, a Snakes novel, a Titan novel, and two sourcebooks. That's roughly two novels a year on average, but he's a much more prolific writer than that: he's also written four Warhammer Fantasy novels, four novels for other properties, two original novels, and a screenplay.
As such, I think an average of two novels per year (health, happiness, etc., permitting) for the 40k/Heresy projects he's already mentioned is a feasible goal.