I'm not going to comment too far on the amount of content the game has - we all know that there is far too little in the way of maps and gametypes, and that we need more of both to sustain the game's lifespan. Personally, I also really want new maps - I'm already sick of the 'remixed' maps, and I've played Halo 5 far less than any other Halo. The variety is just awful, and it doesn't help that they're all in the same bland UNSC aesthetic style. They're dull to look at, dull to play, and discovering that the 'new' map is yet another remix (i.e. hardly worthy of the title of 'new map' at all) has only lowered my expectations further, as it just feels lazy and cheap, as though it's being pushed out simply to fit the promise of extra maps, rather than because it's a genuinely interesting map. I've also complained about the REQ system at length before, so while my opinions on that haven't improved, I'm not going to restate them here.
In terms of actual mechanics - generally, I actually think most of the gameplay is good as it is. Probably the biggest concern of mine is grenades - for something which does so much damage over such a large area, spawning with two and having so many on the map is just too much. They could definitely use a reduction in either spawning number, or overall power, and possibly both, because right now Halo 5 is awash with grenade spam, and forcing grenades back into the role of tactical weapons they had in Halo 3 and Halo 4 would make for much more interesting gameplay.
Next thing would be, something needs to be done about the motion tracker. My personal theory on this is that the pro team insisted it shouldn't be included because it's 'nooby', while the devs wanted it to be the same as it always has been, so in the end they reached a compromise of 18m, which seems very arbitrary. The biggest issue for me isn't even coming from Halo 4's tracker, which had extremely long range, but that it differs from Halo 5's own campaign, meaning for a player coming from the campaign it's outright misleading. It's too short-ranged to really be of any use, and yet it's still there being distracting visual noise, and it's a step back in pretty much every way from where things were in Halo 4. I don't care whether they buff it to campaign levels or remove it, but its current status is worse than useless, and needs to be addressed.
The last thing which really needs to be looked at is the matchmaking system, which while technically functional, is appaling for putting together completely lopsided and imbalanced teams. It's simple fact that occasionally your team is going to be at risk of getting steaked, but I can honestly say that in about half of my matches, I'm literally the only player on my team who's not trading that poorly, and the constant poor matchmaking has outright killed my enthusiasm to play. Players need to be allowed to drop ranks if they're placed too high, solo players need to be matched against solo players, and teams need to be properly split into roughly equal power levels.