GameRager: Sin Episodes needs a sequal but I think they have no money or something. Too bad too...because the lead-in/teaser to a part 2 at the end of the game is pretty awesome.
FlintlockJazz: I believe the problem is that the studio that made Sin Episodes, Ritual Entertainment, got bought up by some casual games developer called MumboJumbo who had no interest in continuing with any of their games, forcing them to instead develop casual games instead. Prior to and after the acquisition many of the staff left the studio, meaning that even if they wanted to they probably don't have the devs to do it anymore anyway.
^^This. Unless the Sin IP is sold off to someone else or MumboJumbo gets bought out by someone with an interest in and the resources to make FPS/action games, the Sin series is dead to all of us. A real shame too, the first episode showed some great promise.
As for the series I'd like to see revived...
Freelancer - was supposed to have a sequel, got cancelled. Terrible injustice.
Freedom Force - was supposed to be a trilogy, now we'll never know how it ends (unless you've read the Phoenix Saga in X-Men).
Advent Rising - goddamned game ended on a freakin' cliffhanger! Was also supposed to be a trilogy.
KotOR - yes, I know there is this MMO coming out, but that is not a sequel to one of the best damn games ever, that is a travesty.
Star Trek: Armada - I'm probably one of the few on this one, but I played the shit out of the first two games, would love to play me some more.
NOLF - the fact that there were only two of these games is another terrible injustice (I don't count Contract J.A.C.K., no one should).
Tron 2.0 - yes, I know about the new movie and the requisite video game tie-in, but neither of them have anything to do with this hidden gem of a video game.
Scrapland - Not really a great game, but a great universe to play in with tons of potential. A sequel or two would get my money.