Guys this is my first post so I'll make it a longer one...
I've played nearly all CoD games, though only CoD2, WaW, MW2 and MW3 in multiplayer. It would take me a whole new thread to describe my experience and opinions about CoD series in general, so I'll just say I'm a huge fan of the series, and I personally enjoyed MW2 the most (best game I ever played besides GTA: San Andreas). It had the ingenuity, the depth and was adapted to every type of player out there, making it far the best CoD out there, IMHO. This is how I'd like MW4 or some other sequel to look like. Feel free to agree, disagree, correct me or improve my ideas. Please keep in mind I'm only talking about multiplayer.
Setting - The game should definitely be set in present day or near future, just like MW2 and 3. It should include modern, real-life weaponry, equipment, factions, places and technology in general.
Player leveling - 100 levels. With new levels players unlock new weapons, equipment, grenades, perks, killstreaks and deathstreaks. You level up by earning XP for kills, assists, destroying enemy equipment, winning matches, completing challenges and calling in killstreaks. Classic challenges would revolve around getting a certain number of kills, destroyed killstreaks, jumping from certain hight etc. (like in previous games). There would also be special weapon and killstreak challenges, e.g. getting a specified number of kills or headshots with some weapon, or calling in a killstreak a certain number of times. They would earn your general XP and influence your leveling, but would also increase your experience with a specific weapon/killstreak, the purpose of which will be described later.
Prestige system - 10 prestige levels. By prestiging everything except your callsigns and emblems is restarted. To motivate players to go prestige instead of staying at level 100, there should be a new group of callsigns and emblems for them to unlock by going through each prestige level. Also, by prestiging, the players would also get a small boost in health and regeneration. By prestiging only once or twice that difference would be almost unnoticable, but by reaching prestige 9 or 10 the power-up would be quite significant.
Maps - Big, but not too big. The largest ones should be the size of Wasteland in MW2, or like the biggest MW3 maps, while the smaller ones need to be the size of Terminal, or even MW2 Skyrise. However, the smaller the map, the more complex it should be, adding more underground passages, cover, alternative ways etc. Still we don't wanna see tiny, crowded maps like MW2 Rust ever again. Also, please no additional maps in DLCs, even if it means a little more expensive game in the start. Some people always buy them, some never do, and you end up having several player communities with access to different maps, and too few players for most game modes.
Game speed/pace and spawn system - Speed just like in MW2. MW3 was a simply too fast. Regarding the spawn system, the preset spawn locations should be removed. It should be left to AI to locate a random spawn point on the map far away from any enemy player. That way things like spawning in the middle of a firefight or spawn trapping would become history.
Hacking - Hacking needs to be prevented at all costs. It ruined every single CoD game so far. Make a hackerproof security system and instantly ban every player who even tries to use hacking software or any type of modding.
Game modes - The ones featured in MW2, BO and MW3. I wouldn't mind several new ones, but I'm quite happy with existing ones. But I'd remove Ground War and offer three types of each mode instead: 6v6, 12v12 and 20v20. That way both the fans of small tactical play and large scale battles would be satisfied. There would also be faceoff modes with 1v1, 2v2 and 3v3.
Classes and customization - Players should be able to create and customize their own appearance (something like in Skyrim). They can also choose clothes, accessories and uniforms, which they unlock by leveling up. The main uniforms would be: several types of regular suits and uniforms (no special attributes), ghillie suit (good for hiding in grass etc.) and ninja suit (great for moving in shadows). Classes should consist of two weapons + a backup handgun, lethal grenades, tactical grenades, equipment, two primary perks, two secondary perks, three killstreaks and a deathstreak. I'll describe all of them in short in the following text.
The main point would be not to reduce the power and diversity of classes (like they did in MW3 comparing to MW2), but to make all classes highly specialised and very powerful. Still, each type of class would be effective in certain situations and against one group of classes and vulnerable to another group at the same time (e.g. sniper effective against AR/SMG/explosive while in cover, but can easily be outmaneuvered by fast and stealthy classes like kniferunners, silenced SMGs and shotguns etc.). That way all classes would be equally "overpowered", so abusing any particular weapon or class would make no sense. The game would be in fact much more balanced.
Weapons - No primaries or secondaries. Players can choose any two wepons of the following categories, even two identical weapons (for having double ammo and two versions with different attachments/proficiencies) if they like: Assault rifles, SMGs, LMGs, Shotguns, Snipers, Machine pistols, Launchers and Special, plus backup Handguns. They'd have to earn experience for each weapon individually by using it during matches and accomplishing special weapon challenges. By earning those points the weapons would level-up. There would be practically unlimited number of experience levels for each weapon, but all attachments and proficiencies would be unlocked by level 40.
The selection of weapons would be greater than in previous games and they would all be real-life weapons. However, that doesn't mean they need to have all real-life characteristics. Instead, they have to make every single weapon feel and behave differently, each with various strenghts and weaknesses. There must not be a "better" or "worse" assault rifle, only the one with more damage, the one with less recoil etc. Every player could choose a weapon with the characteristics they prefer, but having the situation where all players use only two ARs or SMGs because the others are nearly useless shouldn't happen ever again. Weapons, attachments and proficiencies can be unlocked in any way a player chooses (just like killstreaks). They would all be well balanced so why forcing someone who hates SCAR-L use it for a long time just to get his beloved G36C?
Also, there would be no more bling perk or 2 attachments proficiency. The number of allowed attachments and proficiencies would automatically increase as you gain experience with a weapon, with a maximum number of 3 attachments and 3 proficiencies. Of course, you couldn't have two attachments which are placed on the same part of the gun at the same time (e.g. grenade launcher and shotgun, ACOG and thermal...).
- Assault rifles: A choice of 12 to 15 ARs. Two unlocked at the beginning.
Attachments: red dot (different types like in MW3), holographic, shotgun, grenade launcher, heartbeat sensor, ACOG scope, extended mags, bayonet (long-ranged melee), thermal, hybrid sight, silencer, FMJ (better penetration), explosive rounds (better damage)
Proficiencies: improved damage, improved rate of fire, improved range, better mobility, improved ADS speed, less recoil, less sway while aiming, reduced flinching when hit, better penetration, hold breath (no sway), faster melee, reduced reload speed
- SMGs: 10 to 12. Two at the beginning.
Attachments: Same as above, but no grenade launcher, shotgun, bayonet. Rapid fire, akimbo and one-handed riot shield (smaller than standard, but you can use it for defence while firing, can't be paired up with akimbo) added.
Proficiencies: same as ARs
- LMGs: 8 to 10. Two at the beginning.
Attachments: Same as AR without grenade launcher, shotgun and bayonet, grip is added.
Proficiencies: same as ARs
- Sniper rifles: 6 to 8. One at the beginning.
Attachments: AR but no shotgun, GL and bayonet, scope with variable zoom is added
Proficiencies: Extended hold breath capacity instead of "hold breath" (obviously)
- Shotguns: 6 to 8. One at the beginning.
Attachments: red dot, holographic, extended mags, silencer, FMJ, explosive rounds, grip, akimbo, one-handed riot shield
Proficiencies: same as AR
- Machine pistols: 5 to 7. One at the start.
Attachments: same as shotguns, with rapid fire replacing grip
Proficiencies: same as AR
- Launchers: 6 to 8. One at the start.
Attachments: sniper scope (only available to some of them)
Proficiencies: same as AR (where possible), increased blast radius added
- Special: Riot shield, Crossbow (normal, flaming and explosive arrows), Katana (better range than knife and can hit more enemies at once)
Proficiencies: increased mobility, damage, reduced flinching when hit, (for crossbow - same as AR), (for katana - faster swinging), (for riot shield - better protection against explosives)
There's more, but I'll describe that another time