Here's my thinking: the world simply couldn't handle another Metal Gear title not featuring Snake without some kind of counterbalance—an all-Snake, all the time game that drives forward series lead Hideo Kojima's quirky vision of life during wartime. Ergo, with Metal Gear Rising Revengeance—the pointedly out-of-canon entry starring cyborg assassin Raiden on the way—the universe requires more Snake to even things out.
Or maybe we're simply getting Metal Gear Solid V (or whatever the next game will be) because not doing so is leaving money on the table for publisher Konami. That's what a global job posting over on Konami's site would seem to indicate, as the company begins a worldwide drive for "the next Metal Gear Solid" accompanied by the artwork above.
So what's Konami (and Kojima) got up their collective sleeves? If I can play part-time prognosticator, I'd guess that based on the pretty final ending of MGS 4, we're either due for another jaunt back into Snake's past or (more likely) some degree of reboot of the series. These kind of things are always cyclical, and it would leave Kojima and co. room to build a new universe on the foundations of the original. Or, my thinking may be too pedestrian and he'll have something more audacious and out-there in mind for the series.
As for Revengeance, Raiden's hack-and-slash adventure (developed by the Bayonetta team at Platinum Games) still doesn't have a release date, but maybe we'll hear something concrete around E3.