The only micro-transaction system that I have seen that works is GTA Online economy system where you can buy currency, earn it base game matches, and doing contracts. I feel that it could work with the Reach economy system. If micro-transactions are a must then that is the best way to do it. But it go great if maps are free and future transactions can go off of in game currency. The in-game economy could be used to get armor, attachments, visor color, armor effects, maybe firefight character packs (like reach) or alternative announcers, weapon skins, and armor patterns. Locking off colors or emblems will be a bad idea and some armor should be able to unlocked viva achievements or campaign completion. Seriously don't do what Bungie did for Destiny 2, people don't like taking a feature away. Say Base Mark V and Base Mark VI which attachments for those helmets you would have to get viva in-game currency system.
Have weekly and daily challenges to get a good bit of in-game currency and maybe have double xp weekend game mode. Also commendations could make a return and have them unlock a good bit of in-game currency.