In my opinion, semi-new era Halo is the way to go. Take a look at Call of Duty for example. Every game is a different campaign, with slightly different looking weapons. Soon enough most (intelligent) people get bored with CoD. That being said, I think some new intuitive game mechanics and a really good story, along with tons of new vehicle and weapon additions is the way to go to keep both sides as entertained as possible without upsetting either side.