recently finished a halo novel called Halo: Grass lands( which takes place after the events of halo 3, 2553) and to say what I need to say shortly, There might be a possibility you fight elites, there's a civil war within the elites, half want to be allied with the humans(arbiter half) and others who want to kill all of humanity. meanwhile there is this prototype U.N.S.C. ship call the infinity. which has extraordinarily advanced technology on it (forruner in origin) it also has Spartan ivs on it, which is largely unknown, not much is known on it, I do know that they take adults now, not kidnapping six year olds. Some of the Spartan ivs are spartan ll survivors from the battle of onyx. (spatan ll= Kelly, Fred, Linda, Naomi) spartan llls= Ash, Lucy,Tom, and I think one other. The U.N.S.C. has also gotten their hands on some engineers from direct forrunner decent. it would make sense with the infinty and spartan ivs. the remaning spartan lls think john(mc) is dead, and are called to go investigate a weird part of the universe (where john lands in the teaser) they extract john via infinity, and that's all that I was capable to peice together tl;dr, forruners might be in halo 4, might fight elites, spartan 4s confirmed, Spartan lls/llls might be in game. and new ship that might play major role.