At present, the profile settings on Halo Waypoint let you select your Spartan from Halo: Reach, and Halo 4, or Halo 5: Guardians and display that as your forum Avatar… problem is, the servers for 2 of those games are being taken offline later this year, at which point I assume we won’t be able to use them anymore.
The question is, why can’t we use our characters in MCC as our forum avatars just like the original games? Update player service records for Halo: MCC to display renders of each players custom Spartans, Elites, and ODSTs across all 6 games (+Halo 2: Anniversary Multiplayer) and then give us the option to select those as our forum avatars. Naturally, the renders should update whenever we change our characters armor in-game.
While we’re on the subject of MCC avatars, we should also be able to select our characters as our avatars in-game rather than being restricted to the static images we currently have in MCC.