With Gears of War, I'm happy with how the franchise ended. There's no real need for them to continue it, there's nothing left to try and discover, no mysteries, nada. But they are making a new Gears game. Whether it's a sequel or prequel, it doesn't matter: It's unnecessary (That's not to say I'm not interested, I'm a big Gears fan).

With Halo it's different, because there's always something left unexplained or somewhere unexplored. This is why MS can continue to make not only games, but comics, books, etc to continually expand the universe. The key is to keep introducing us to new people, events and places that we want to further explore. Keep making us hungry to travel deeper into the rabbit hole.

I think 343 have been brilliantly successful in doing this so far (see Janus Key; Halsey revenge). As long as they continue, Halo has a bright future; at least as far as story goes.