I think there's three core things lacking in Halo 5 at the minute (besides Forge but that's coming in a couple weeks) holding it back from reaching its full potential. I personally think the core gameplay, the shooting, the movement and all that jazz is the best it has been in the series for me..
1)
The amount of content - We need more maps, which we know are coming. I just hope there's enough of them, and they are quality maps. For both Arena and Warzone. While the PR map count was great to hear, when you boil it down, things are stretched pretty thin in each mode. The good news is, they are in the works, and they are all going to be free. And hopefully community cartographers can pull off some awesome work with Forge 5.0 themselves and that can get added in the future.
2)
The lack of features (from prior games) - All the little things that Bungie built on over time, and even 343 in Reach that made Halo what it was. All of the gamemodes that we are currently missing like Griffball, Oddball, Races etc. Along with playlists in multiplayer. Weekly and Daily challenges to keep people coming back. Rewards in all modes to entice people to replay campaign (Along with campaign scoring) and feel like everything you were doing had extra value. QoL stuff like seeing what people are playing through the in-game lobby. And all the other stuff I can't think of off the top of my head.
3)
Not having all things for all people - In the past, regardless of my mood, I have had something to do in a Halo game. Whether I wanted to push myself and try and rank up, or kick back and have a blast with some friends. In Halo 5, at the minute it's either go pro in Arena or play Warzone. I enjoy both but Warzone can get super stale after a couple hours, and sometimes I don't want to go MLG pro in Arena. I think a dedicated PVE mode is vital, especially in today's market with Destiny and CoD with Zombies etc. It's the perfect middle ground for people who want the awesome gameplay without the headache.
I mean, just thinking back on the campaign, I'd be more than happy to have a new and improved Firefight with REQ's in campaign play spaces if that's the best they could do with resources for the time being. I'm sure there would need to be some tweaks to allow vehicles but hell, some of the spots in the campaign were gorgeous and the level design was excellent. Just imagine battling waves here:
Or even just go for Warzone Firefight. Why not.