IGN delivered an interesting quote in yesterday's Scott Miller interview, regarding the direction 3D Realms are taking with the new projects they're working on:
While Prey pioneered the portal concept, Valve's Portal took it to an all-new level. It really shows what you can do when you focus on a single core cool gameplay concept. So, while Portal impressed the heck out of us, it has also inspired us to return the favor to Valve and hopefully leapfrog them as they did us. We believe we have new ideas for portals that will keep the portal race interesting.
That's an interesting statement: "The Portal Race" could be an awesome period of FPS development history.