As much as I cant and wont play Halo, I would actually have to argue that Halo is greatly innovative for what it is, and concidering the state of a lot of FPS games, its very very well done that and it actually has a plot
CoD is just rolling it out for money, people will but even shit CoD games, but Halo is a bit different because of the mythology and universe behind it.
The multiplayer is also still and constantly developing, they aren't just taking the same concepts again and again, they are always innovating it to make it better.
If you play it and look at it you realise this that with halo, the FPS is just the mechanics, is the game and the universe that makes it a well done and engaging game, and if you can look past the stigma of FPS multiplayer games, there is amazizng content in it. Yes its gone a bit far, and fuck the fanboys and their pushiness of it, especially the teenagers that thing CoD and Halo are the best games in the world, but Halo 4 was innovative for the series and thats what counts.
As far as the actual universe though, I was Haloed out after my day at uni (I go to a game design uni) and it just makes me more glad I havent played it.
And on the matter of sequals in FPS, I personally loved bioshock 2, and cant wait for inifinite.