This is the Hornet Pilot Helmet right here: https://www.halopedia.org/images/d/d9/H3-UNSCVTOLPilot.png
This is the NMPD Riot Helmet right here: https://www.halopedia.org/images/d/d7/H3ODST-NMPDCop.png
These are not new, they have existed within the files for Halo 3 and Halo 3 ODST for 15 years now. adding them to the characters in the game is literally on the same level as both Bug Splattered Buck and Injured Romeo, as well as what Bungie themselves did when they had Johnson show up in ODST’s campaign (it’s literally just a default marine body loading the ODST Johnson head, which is distinctly different from the Halo 3 head model).
the point I’m making is that it’s significantly easier than what your suggesting to just utilize the assets readily available and integrate them into the game, as they don’t even have to pull the assets from an engine completely divorced from it or rework an entire system to make it work. Heck, Characters like Stacker are basically set up and ready to go, further streamlining the process. just about the craziest part would be adding that H2A ODST Helmet, but even then, that engine is close enough to Halo 3/ODST to render the point moot.