Chasing trends will only result in Halo being behind other as you can't be first if you're copying, other will always be a step ahead since they are the first to make the move.
Sure, having just classic Halo and a pure arena experience won't put Halo on top of the trends, but, going back to classic Halo could help the game improve on other areas since it doesn't have to focus on player abilities, since the basic gameplay has already been tested and approved.
If Halo wants to be the top dog once again, it needs to build since on its own that other game cannot afford and give to the public.
For me, Halo's strengths have always been its base gameplay, its variety and its narrative focused rich universe. They should combine this into a new game mode, something Halo already tried to some attempts with Invasion and Core Reactor Firefight in Reach, Spartan Ops in Halo 4 and now Warzone with Halo 5.
As for myself, I would kill to get something like Invasion mixed with Warzone for like 12v12 players.
Having narrative driven gametypes & maps, where Spartans need to move from point A to point B with a cinematic introduction, unlocking the map the more they are moving forward and getting better weapons (Invasion from Reach).
One team would be the Spartans, gaining better weapons, the other team would be The Covenant, unlocking MOBA like AI units like Grunts, Hunters & cie they could deploy on the map to fight against the other team (Reach's Core Reactor Firefight).
Maps would explore different scenarios within the Halo universe and unfold around a story set for seasons/chapters and could change following the numbers of victory from each side (Human versus Covenant) (aka Halo 4 Spartan Ops).
I mean, if you look at what
the community achieve with Halo Custom Edition, there are ton of ideas to grab and polish for a new and complete Halo experience.
We could more into this, but that's the basics I would like to see for a new experience within the Halo universe, without having to rely on trends and keeping the basic style of Halo gameplay.
And, on top of that, you keep the Arena (4v4) and other modes like BTB (8v8) allowing Halo to keep some variety as it should.