Gears would be cool because it would be a fresh type of sci-fi. Forza would also be interesting to watch, particularly the rivalries that could make the show somewhat dramatic. State of Decay would be kind of redundant as we already have The Walking Dead, and any show that attempts to compete with it is doomed to fail.
Honestly, I'm kind of annoyed that more game IP's haven't been adapted into shows. We all know that video game movies suck horribly, why not have shows adopting these worlds which are already really interesting to millions? It's like doing half of the work of having to pitch a pilot for you.
"I refuse to prove that I exist" says God. "For proof denies faith, and without faith I am nothing..."
While I enjoyed the hell out of State of Decay, and would love to see it in serial form as a series, I have to agree that the world just doesn't need another zombie series, what with The Walking Dead and Z Nation in production for SyFy. Forza, on the other hand, could be spectacular given the right host(s). Top Gear USA is trash, and there's no doubt that the folks behind Forza obviously love cars. Plus, they've got to have countless hours of video footage in the bank already with all the testing they've done. I'm not so sure about Gears, as I've only played through the first title, but I could see it being a more gritty version of Falling Skies, which I think I'd watch.