Integrate file sharing & Bungie favorites into Steam Workshop so everyone can upload/download clips, screenshots, game-modes, etc. It would be a lot easier to use the workshop rather then doing everything in-game through user profiles like on the console versions.
Also some kind of support for mods would be awesome.
Think of the injection of life and total playtime games like Half-Life, Counter-Strike, Unreal, Skyrim, Stalker, and many others have gotten because of mod support.
Definitely make sure the Windows Store & Steam versions have a linked player-base. This is an absolute must.
Extra support for cross-play with consoles would be nice too. It could be handled via separate Ranked/Social playlists, game-modes, or through Opt-Ins. Similar to Rocket League and Fortnite.