Brutal Legend was pretty good, but not the sort of "good" that ensures an instantly greenlit sequel. If it takes as long as it did for Psychonauts, which was also "good" but far from a runaway hit, it'll be another four years before it's even announced, and maybe three more after that until it actually arrives. If, however, the recently-announced Psychonauts 2 does well, then maybe—just maybe—it'll happen a little quicker.
There are no guarantees in this world, of course, but Double Fine connected the fate of one game to the other in the Psychonauts 2 FAQ on Fig. "We've always said that we wanted to make sequels to both Psychonauts and Brutal Legend," it states (and isn't lying). "We can't make any promises but if Psychonauts 2 is a success for us, then perhaps Brutal Legend 2 will be more possible?"
It's hardly uncommon for developers to say that they'd like to revisit their old games, or to connect a hypothetical future game with one that's in the works. But in this case there's more of a practical angle to it, because Double Fine is in the midst of raising $3.3 million to help fund Psychonauts 2. "Can't make any promises" is clear enough, but there's clearly a little bit of cart-hitching going on here.
It hardly seems necessary: Less than a week after it kicked off, the Psychonauts 2 Fig (which I guess is what we're calling it) is approaching the $2 million mark, and seems set to meet its goal with relative ease.