I thought the ending was well executed. It was supposed to be dark, and frankly you don't always get what you were striving for. The fact that your main objective essentially failed only brings out the idea that things don't go according to plan. Halo 4 brings in a strong sense of reality that is hard to swallow for some.
They say Halo 5 will be darker, so don't expect Cortana back in the next game, if at all.
I'm thinking Cortana will be back in Halo 6. The only issue is how she could possibly come back. I see no evidence of being able to rationally restore Cortana, so 343 would need to pull some magic. But let's say there's a way to restore her. After they do it, considering AI's go rampant, they might (and this is a big might) make her human, or "give her life" in some way.
My reasoning? Why else would 343 introduce the Composer? It's an artifact that can turn life to data, and data to life.
Thoughts?