Keep in mind that when we compare controllers it's not just the objects that are at our hands, but also the technical details of how our manipulation of those objects translate into aiming. Keep in mind that the goal of the aiming control is to position a crosshair onto a target. The essential difference between a game controller and a mouse is:
* A game controller receives directional input from the object in our hands i.e. we press an array of buttons to indicate the direction in which to move...we must wait for when the point where we wanted to go has arrived then release the button.
* A mouse receives positional input from the object in our hands i.e. we simply move to the desired point.
Thus, a game controller has a built-in requirement for a set of complex steps in order to achieve aiming while a mouse is a direct aiming device. This doesn't mean that someone using a controller can't aim as well or better than someone using a mouse...there are some very talented FPS controller gamers that can do well against a mouse gamer. However, the average gamer will find it much easier to learn and play FPS games if they use a mouse for aiming. A fact that boggles my mind as to why game developers insist on designing FPS games for consoles when consoles are the worst platforms to play them on.
So, what will cross-platform do? Provide the developers with some technical challenges...that's all. The server configs will definitely want to include the ability to restrict games to players using game controller...and also restrict games to players that don't use auto-aim. Mixing games with players that use mice against players not using auto-aim with game controllers along with players that do use auto-aim will be a disaster for everyone...as it has before.
What really needs to happen is for game consoles to standardize better game controllers.