Here are my responses to what you put:
Sprint - This will be difficult to please everyone. I don’t have a HUGE problem with sprint, but personally I just don’t feel the sprint animation is necessary at all. Just up the base movement speed. I like having my “gun up” all the time, that feels more like Halo IMO. Sprint in it’s current form really changes the game in every area as we know.
Spartan abilities - I would keep clamber, but everything else goes. I would then make things on the map like power ups, called…oh I don’t know…equipment or something and have things like thrusters you can pick up that allow you to do things maybe like Ground Pound, Spartan Charge etc. Maybe you have them till you die, maybe you just have so many uses, or many you have a “power level” which allows you to do whatever until it runs out, I don’t know. All I know is Clamber is the only thing I feel that is good.
Free dlc, should it be in H6 - Free DLC is a great idea, we all like free. 343I just needs to make sure the free content isn’t Forge maps, remix maps, emblems or the 5 version of some kind of gun. If DLC was all developer made maps, new game modes we’ve never seen, and cosmetic stuff that actually looks different. I think everyone would be pretty happy.
Game modes for veterans - IMO if the game is done right, this isn’t needed at all as the game will feel and play like how a Halo game should. Like Halo 1-3. Not BE Halo 1-3, but feel and play like it. Big difference there.
Time to kill, the same or more - It should be somewhere between Halo 1-3 IMO Halo 5 isn’t too bad in this area I feel.
Playable elites, Where the -Yoink- are they? - They could easily put in Elites I feel. They could have an invasion type mode like Reach had, or do 4-4 where it’s either Spartans vs Spartans or Elites vs Elites…then have a social mixed playlist of Spartans vs Elites if they want. There is many things they could do here and still make the matches fair I feel.
User interface, good or bad? - The UI in Halo 5 is one of the worst UI in any Halo game and maybe one of the worst UI I’ve seen period!!! Yes it has gotten better but still. The UI needs to be some kind of mix of Halo 3/Reach I feel. There needs to be a REAL pre game lobby where you can actually, you know, mute single person like we could 10 years ago and a post game report. The post game in Halo 5 NOW isn’t too bad.
How to make H6 campaign better? - This could be a very long topic, but at the very least 343I needs to assume the player hasn’t read any comics, books etc. Nor SHOULD the player have to read these things to FULLY understand the story and what is going on. So they NEED to explain things. As much as a lot of people like Halo 4 story, I still feel Halo 4 was very poorly TOLD. Yes parts were done well, but other things were so poorly explained. Halo 5 was WAY worst in this way. Short version, just explain things and as I said, assume the player hasn’t read any books or comics.
REQ system, should it stay or should it go? - I’ll be very surprised if REQs aren’t in Halo 6. REQs have made 343I/MS a lot of money so they are here to stay for the moment I think. So, stop making so many versions of guns and vehicles that are more less the same, have more unique looking armour, helmets etc and such that actually look different and for the love of god, get ride of emblems in REQs!!
I’ll add, Map design needs A LOT of work in Halo 6. Maps need to be more like Halo 1-3. They actually look different. Snow, jungles, ruins, etc. Not just Human buildings…Maps also need to too have interactive elements like H2A. Shooting chucks of ice down, making the water flow down, putting up a huge shield, etc. things like this!! H2A maps are the best maps IMO. Looking, playing wise everything!
There needs to be social and ranked playlists off the bat. Like how Halo 3 did it. Social 4-4 slayer is a must. Ranks name should be Military ranks, not this garbage of Gold, Onyx, Platinum, Diamond stuff. Rank resets also need to go!!! People want to work towards something, not have to start all over again every month. I know many people who have flat out quit Halo 5 because of this. At the very least, make seasons WAY longer and show the players highest rank in that playlist beside there name when in ranked mode.
I know there is other stuff I can say, but I’m just too tired right now lol