well they are suppose to be co developing halo chronicle with peter jackson wingnut studio...... and i remember to have read somewhere that they were keeping there engagement on that matter even after they left MGS....
and about people talking about halo 3 I honestly think we won't see another halo FPS in long while....doing XIII version of a game is a Sony trend lol :) MGS proved they can come up with new IPs and I don't know if you noticed but most of them are just planed to be trilogies....