The title is pretty self explanatory but to further iterate what i mean and why it is important, ill explain...
The whole point in cross-platform is to connect people! most people cant afford to buy more than 1 platform because they all cost around £400-£500 (including PC) so one is enough and most people will only buy one, not everyone is rich. The same goes for buying copy's of the game on said platform, especially since it costs like £50 per platform these days, not to mention DLC.
As a result of most people not being loaded with cash, it can become a nightmare to find out that half your friends bought the PS4, a quarter bought the PC/SteamMachine and the other quarter the XboxOne. Therefore you are limited to a separate private network, to who you can play with and how many friends you can play with at once.
In my experience, when talking about purchasing multiplayer games... the Best Selling point to me is, "how many of my friends bought the game?"
Gamer's are alot more social these days, gaming can be used as a very powerful medium to socialize even more so than chat boxes alone, as you have a 3D character in a 3D multiplayer space, it gives the players a sense of presence, which allows them to PLAY or be Playful in nature in ways that are not possible on facebook... So anyway, the option to connect to everyone would be a massive selling point to the game, if nobody no longer has to worry about what platform they are on, I swear hands down more people will buy this game.
'Gamepad Only Servers' is an idea i thought of, to pretty much blur the line between consoles and PC even further.
The way this could work, is there could be a filters on the server browser or matchmaker called 'Gamepad Only' or if that doesn't go to your taste, we could have little gamepad icon's for PS4,XboxOne,PS3,Xbox360 not that these gamepads have any real advantage over the other. When you join a 'Gamepad Only Server' on PC... vital keyboard and mouse buttons such as 'W' 'A' 'S' 'D' etc. could be temporarily un-binded... in the same way when on CounterStrike: Source, you can type in the console UnbindAll.
'Keyboard + Mouse with Gamepad Servers' could still exist except auto-aim/aim-assist should be turned off perhaps if the Gamepad users have a slight un-natural advantage over the KB+M users.
'KB+M with Gamepad Servers' could also exist on consoles, since you'll be programming the binds for KB+M for PC, why not port this information to consoles? and give them a taster of what KB+M is like, most people own a USB mouse and keyboard so why not.
If Cross-Platform cannot be achieved on all 3 platforms Xbox, Playstation, Steam... then we should still have 'Gamepad Only Servers' on PC since this will mean there are no differences between each platform, meaning people would be more inclined to buy to game if they prefer or just like gamepads but own a PC.
Local Split-Screen Multiplayer could easily be integrated into this Idea. Especially with 'Gamepad Only Servers', Split-Screen with Online! would be a brilliant selling point for the PC version, Split-Screen should never have been removed from the COD series on PC in the first place, because of technical limitations being far less depending on your hardware. Don't forget 4K TV's and Displays can only be used with PC, another reason why Split-Screen is so much better for PC.
As for framerate differences in cross-platform... COD on consoles runs at 60FPS anyway which is super nice, so it really is not that much of an advantage to have 120FPS on PC, it does seem smoother, however it is a minority of people that own 120hz monitors so most people on PC will be running 60hz anyway.