The wide swath of available positions includes general planners, level designers, various programmers, and CG designers. While the level designers and programming positions may be building the crew for crunch time on Metal Gear Solid Rising, the planner openings and "next generation" verbiage imply that more Metal Gear Solid games are in the works.
It's hard to say how a next generation MGS game might look. Solid Snake quietly retired in Metal Gear Solid 4, and Kojima Productions has been highlighting other heroes since. At this point the next game after Rising could star Raiden, Big Boss, or someone else entirely.