Boy am I glad I found this thread! I was about to make my own; I wasn't sure quite where to look for people sharing similar sentiments about this game mode!
Reading through the postings already made, there are some things I agree with, like releasing the ODST Firefight experience with the PC drop. While I think it was a huge missed opportunity to release a complete ODST package back in 2015, I could understand wanting to play it safe, and just focus on one mode (unlike their experience with releasing Halo 5 at the time).
Then came Reach - it would've been nice to pair the releases of both Firefight versions, but again for the sake of the fidelity of the Reach re-release on MCC, just focusing on Reach was the right thing to do. Especially as it was a dual release on the MCC and PC.
But now we are looking down the line at the next titles: CE, 2, 3, ODST, 4. There were some great ideas mentioned here about moving all the Firefight and Spartan Ops game modes under the Firefight tab with some of these releases, and I think that'd be great. Though I'm not sure about matchmaking for them, mostly because of what Strictcleave112 had mentioned before. I think these modes would be fine just as local game types that support cooperative play, and leaving Reach Firefight to be the only one with matchmaking. We know how 343 has struggled quite a bit with MCC's matchmaking lifecycle since 2014. No doubt they've made tremendous improvements, but I don't think that's a necessary burden, especially since the original modes never included matchmaking.
But, I also think there may be something to the shared images with Johnson in Firefight against the Flood - I think and hope that it may come with the release of ODST to the PC, or at least soon after/just before. While tough, from my understanding it's not like they're building each of these games from the ground up for the PC. They don't have to simultaneously release with Xbox, and they will get plenty of practice with only the base titles in CE-3.
Unless my estimations of the real gravity of the work for these projects are wrong (which honestly they easily could be), I don't think there is much of any excuse to not include ODST Firefight around the PC release of ODST. Especially given the modding teasers from the previously mentions post. It's my understanding too that teasers usually mean something relevant is in the works.
And a last note on Firefight for Infinite, I would think that the best way to advertise for that, would be to first release it on MCC - as some people had similarly mentioned. It' the perfect testing ground for traditional content, and everything in Infinite we've seen so far has been calling back in some way to previous issues in the community, mostly revolving around design choices. (Again, my fallible understanding of the product). I don't think they'd dare to really focus on ODST yet until they really gave it a test run in MCC. But that said, Infinite releases this year (assuming no delays or unexpected issues). If it's not in the game plan already, I don't think 343'd be confident enough to pull a Halo 2 and be adding things to the game pretty much up till the week before release.
And I'd be totally down for more Firefight achievements, including the return of Endure!