As far as clones of video games go, I think of a clone as something along the lines of someone making a Pac-Man game that isn't really Pac-man but tris to get that same kind of feel to it. Usually clones are made as a cheap ways to cash in on or make a freeware version of something original. Dark Forces could be considered a clone because it was made back when Doom/Wolfenstein were the original FPS and there wasn't really anything else like it, but I think of Dark Forces more as one of the pioneers of the FPS genre, afterall I don't think it really tried to emulate Doom so much as it tried to be its own thing. Certainly it was inspired by Doom in a "Hey, that is a great idea, what can we do with that concept?" sort of way. And for Halo clones, maybe Halo had a few original ideas and others picked up on those, but I think we are past the point of cloning when it comes to FPSs. In all honesty the FPS genre doesn't have much variety, there are countless titles, but they are all more or less the same. We need as many Half-Lifes and Halos as we can get. Games that take a step beyond and try to add something new, it is to be expected that newer titles of the same genre are going to pick up on those things. I don't think that makes them clones, I think that is the normal course of evolution for video games.
As far as clones of video games go, I think of a clone as something along the lines of someone making a Pac-Man game that isn't really Pac-man but tris to get that same kind of feel to it. Usually clones are made as a cheap ways to cash in on or make a freeware version of something original. Dark Forces could be considered a clone because it was made back when Doom/Wolfenstein were the original FPS and there wasn't really anything else like it, but I think of Dark Forces more as one of the pioneers of the FPS genre, afterall I don't think it really tried to emulate Doom so much as it tried to be its own thing. Certainly it was inspired by Doom in a "Hey, that is a great idea, what can we do with that concept?" sort of way. And for Halo clones, maybe Halo had a few original ideas and others picked up on those, but I think we are past the point of cloning when it comes to FPSs. In all honesty the FPS genre doesn't have much variety, there are countless titles, but they are all more or less the same. We need as many Half-Lifes and Halos as we can get. Games that take a step beyond and try to add something new, it is to be expected that newer titles of the same genre are going to pick up on those things. I don't think that makes them clones, I think that is the normal course of evolution for video games.