Halo: The Master Chief Collection is changing its progression system for the upcoming Season 2 update. Developer 343 Industries announced that it will soon begin to allow people to purchase "seasonal items." [Update: following the publication of this story, Microsoft removed the word "purchase" from its blog post, so it seems microtransactions may not be coming to MCC]
MCC currently has a battle pass-style progression system that players grind through gameplay to unlock new stuff.
"We also heard feedback that players want more choice in the order they unlock items, so we are adding the ability to purchase seasonal items in a given tier in any order that players want," design director Max Szlagor said on the Halo website.