I have played a LOT LOT LOT of Halo. I really think the core MP in Halo 5 is pretty excellent. It's the best balanced FPS on console. Been playing it regularly for 3+ years now and it's still exciting and interesting and challenging and, at times, funny as hell.
TTK is fine IMO, all weapons can be useful, the game is fair and (in its current state) is well balanced. Aim assist is fine - - makes the game accessible, yet there's plenty of room for skill to be rewarded.
I really don't have core gameplay issues, but there ARE features I would like.
1. Treat BTB as a valued, important asset from the outset. BTB has been the also-ran this entire cycle, and it's easily my favorite playlist. Vehicles are integral to Halo.
2. I loved Warzone for the first few months. So many wonderful toys! The variant weapons and vehicles are SO fun. Then the hardcore had WAYYYYY too many REQs built up and it got impossible to play casually without a sizable geared squad. Now, REQs made them a lot of money (supposedly funneled to HCS) and they aren't going to leave that on the table. All the 1st party games WILL have some sort of DLC/cosmetic/extra $$ hooks. In return we get the game for free. But maybe cap the total REQ spend per match perhaps? So you can have all the uber-weapons you like, but you have to ration them. You only get 100 REQ points total or something. Afterward you only get normal load outs. Something along those lines.
3. MAKE A WAY TO MATCHMAKE FOR CAMPAIGN. This made me so angry. The first few weeks, it's easy to grab whomever and run co-op missions. The campaign is very fun to play with teammates. It is NOT fun solo on higher levels (it's actually quite fun on low levels, just wrecking people). If you are going to FORCE the campaign to be co-op at all times, there MUST be an easier way for EVERY player to field a team.
Even if 343 goes back to a standard Chief-against-the-world campaign mode with co-op like H3, that online party system needs to be easier. (And while you're at it, find a way to get it in MCC somehow. That'd be AMAZING.)
4. How fun would it be to steal back some ideas from Bungie and have some sort of Halo raid? In whichever way - - Firefight, Warzone, a raid, Spartan Ops-ish idea, whatever - more grouped co-op PVE please.
5. Add a new enemy. Even if it is flood-form Promees. Not a huge fan of the flood, but can't get to a 3rd game with no new enemies. I think they know a 3rd game that is Promeathean-heavy will not work.
6. In campaign, we need a few moments where we feel AWESOME. Halo 5 didn't have them. It made sense given the scale of the guardians and the wardens - - we were supposed to feel small up against them. But I never felt especially super-cool-amazeballs-look-mom-i'm-a-spartan in H5. The only time that really occurs IMO is the opening cutscene.
7. Playlist - - I should ALWAYS be able to play objective-based games if I want to. Quality Halo play is/always has been/always should be about team-based strategy. I get some people just wanna shoot people in the face all day every day. They can play slayer. I don't much care if the CTF/Assault/Oddball/KOTH/Headhunter/etc. playlists are lower populated. Offer one at all times.
I'm sure I'll think of some more. Maybe I'll go play a little and see if I think of anything else.