They could have Halo 5 being about saving Cortana in some way, and in the end, there would be like a cliffhanger showing The Arbiter. Then Halo 6 would have a campaign leading up to the final battle between Master Chief and The Arbiter. Does this seem any good to any of you?
But this is also a great plot for Halo 5. Didact survives, Masterchief finds out, Cortana died trying to kill The Didact, but it was all in vain. Masterchief seeks revenge, and goes straight for The Didact. Chief kills Didact, and the game ends. But a cliffhanger about Arbiter shows. And Halo 6 is about The Arbiter, wich leads up to the final showdown between Masterchief & The Arbiter.