Halo 5 has three key things that keep it alive.
The Customs community
Matchmaking
Clans
I want to suggest features that would complement clans.
1: Creating a more detailed customization system, and being able to send people what you create and them being able to use it.
2.: A customization system that resembles that of Halo Reach not only is that known as the best system but we can tell if they can get the armor or not.
3. Custom games max limt of players increased to 32.
A few personal things I would like to see:
Return of split screen
Gifting requisitions not with real money.
Gifting sliver packs.
A nice armor for max level