I’m not too sure about adding marines to playable mix, but it would be cool to get your own marine squad and maybe differentiate them this way.
The game feels better and more familiar with a Spartan as your player model rather than the Goldeneye/Perfect Dark approach of just putting you into a character model of your choice.
There could be marine centric armor coatings though.
On a serious note. I can’t imagine 343 allowing Spartans to dress up in BDUs, but if they can pull something off like the Nightfall armor set from Guardians I can totally see it happening!