In an update posted on Halo Waypoint, 343 states: 

As we had mentioned before the holidays, the team continues to work on upcoming Halo: The Master Chief Collection updates to improve the existing experience and add new content. We’ve continued this work through the holidays, and have multiple content updates in the works. Our next content update, which will address UI and a variety of other areas, is currently on track for delivery next week. We will provide additional info regarding the exact ETA and final details of the upcoming content update once the certification process is completed.

Additionally, this Thursday, we’ll be updating MCC to include new achievements for Spartan Ops, as well as some additional ones for multiplayer. These will become available this Thursday, 1/8.

Improving your experience with Halo: The Master Chief Collection remains our top priority, and we thank you for your continued feedback and reports on community forums and elsewhere as we prioritize updates for each CU. We will continue to provide the latest information via this thread.

Whew, how many achievements will this game have when it’s all said and done??? LOL

-Sal