I'm expecting 4 things from Spartan Ops.
1. Jul ' Mdama. I really hoped he'd have at least some role in the campaign but he never showed. I like how he's the big man in Spartan Ops though, arguably a free campaign DLC.
2. Didact. Likely not until the very very end of all the last Episode will he make an appearance, though I would not be surprised if he influences things soon.
3. Cortana. If you read my Composer theory, it's highly likely she isnt just "gone." I believe that the artifacts we've been seeing and Halsey's contact in Ep 4 are all Cortana-related, if not her to begin with. In order for there to be more Halo games after this one they'll need Cortana to stay important to the story, even if not present.
4. Flood. Come on......we know they're out there, we know it's probably only a matter of time...... It'd also be one hell of a cliffhanger for the final chapter. I personally hope they never show but it's always a possibility.