After the recent 343 announcement, it seems like Elites are quite likely. They did say that they do plan on focusing on cosmetics (referring to armor), however this may also mean character models. They did also say it's the fans' universe and acknowledged their mistakes, which may mean that Elites could be reintroduced alongside 4 player splitscreen and Reach customization. Simply speculation, of course.
Sangheili (Elites) In New Trilogy Matchmaking
OP Mat Man 412