Things for R3:
Ironsights should be true ironsights.
Shooting from the hip should be more accurate, especially for the bullseye, meaning the reticle can't be super wide or spread way too fast.
Weapon damage range for standard rifles should be much further.
Lose the big XP letters in MP.
Give standard rifles more impact, not by fucking up the recoil but by giving individual hits more impact in terms of reaction and sound.
Enemy AI needs to be smarter in terms of using cover and flanking, R2 AI in execution seems much dumber than R1 AI even IF routines might be more advanced.
Level designs should be much less linear, more interesting and allow for both the player and the enemies to flank each other.
MP levels need to be much more varied and iconic, there's nothing memorable about R2 MP maps.
Vaulting over waist-high objects should be a given.
Grenades cannot be like throwing balloons.
Take time to write a story, take time to flesh out the universe, take time to build the game, the one and a half year cycle is going to burn out the IP, you won't even have enough time to tweak and tune SP levels, let alone MP level.
Find a protagonist, MAKE HIM INTERESTING, STICK WITH HIM.
Make interesting supporting characters, if you're going to have teammates, don't make them throw-aways like in R2. Cartwright was way more interesting than all of Hale squad combined.
Focus more on QUALITY of each mode than actually having so many modes.
Do away with the AI general in scrimmage, give each team various objectives and let players organize into squads themselves like in the first game, strictly following assigned objectives one at a time is BORING, no more "go to this place and hang around for 1 min" objectives.
Visuals: HDR, self-shadowing, reflective water, environments that actually cast REAL shadows, get all the art in properly for every single level, or toss the level out. R2 MP and SP maps look rushed and unfinished. QUALITY OVER QUANTITY.
Lose the two weapon system, it doesn't work for Resistance weapons because there are too much differences in range and impact.
Either bring back weapon spawns or have ammo boxes, don't let the player run out of ammo.
If you can't make 60 players interesting then go back to 40 because R1 at 40 is actually better organized and team-oriented.
Boss fights need to have multiple patterns, weakspots and usage of environment.
You don't need to have cheap enemies, they make the game cheap.
If you're going to have enemies that can cloak themselves, DON'T MAKE THEM ONE HIT KILL, make them smart, make them able to use the environments, make the cloak fail progressively as they get hit more, it's fun to play hide and seek if the hiding and seeking lasts more than 2 seconds for either the enemy or the player.
BRING BACK NATHAN FOUTS AND MIKE STOUT.