Haydn Dalton, Lead Programmer at Vigil Games is ready and raring to go for Darksiders III. The only problem is, Darksiders II hasn't launched yet, and he needs a hefty amount of sales to greenlight his next project.
He specifically noted that if Darksiders II sells "4 or 5 million units" (give or take a million of course!) THQ President Jason Rubin would most likely ok a sequel. If it doesn't, Vigil may be assigned to a new, more lucrative project.
Dalton also states that even if Darksiders III is a go, Rubin would probably want them to "reduce the scope so that we can make it even higher polished." Considering the sheer undertaking that is Darksiders II (approximately four times as big as the first game, according to the developers), I can easily see that being a reality.
As for myself, I wasn't fully sold on the game until I found out there was going to be a Diablo style hardcore perma-death mode a few weeks ago, as well as a fully fleshed out New Game+ mode, which the first game lacked.