343 has said crossplay, input-based matchmaking and custom game browser will all release for Halo: The Master Chief Collection in 2020.
Meanwhile, custom game browser, per game graphics options, and mouse and keyboard support for Xbox will all release together.
Perhaps the highlight is crossplay between Xbox One and PC. 343 said crossplay, input-based matchmaking and server region selection will all release together.
Other points of note in the update: ODST weapons are coming to Halo 3 – and vice versa.