If you like big RPG's and are looking for something new to play, check out KoA. The combat system is pretty good, the world is big and diverse, and if you don't get bored of it, you'll have a good time.
Unfortunately, it's easy to get burned out on it. The main story is not that long, though, so you can easily just power through if you want it finished.
Haven't played the DLC, but at the rate of which I grew bored of the base game, I wouldn't recommend spending money on that, unless you just want to pad your numbers.
With Halo 2 Anniversary and possibly Halo 3 and 4 ported to the Xbox One, is this even a possibility anymore?
I think it's still possible. Hell, since they're working on ports to the Xbone, it might be even more possible those upgraded ports end up on the PC as well. At least, that's what I hope.
I'd buy the shit out of them, even Halo 4.