Here are some mech pics for inspiration.....
I think the GM should essentially choose the level of tech, style of mechs and control interface that everyone has, to keep it sort of standardized.
As for interfaces- I would say a direct interface between the mech and the pilots body/mind would have great advantages, particularly for humanoid bipedal mechs (as instead of messing with controls the pilot simply walks, and instead of the pilot walking the nerve impulses cause the mech to walk) it enables the mech to handle just like a human being (Except stronger and faster) The pilot would probably also feel what the mech feels, i.e. pain when the mech is hit, dizzyness if the mechs gyro becomes unbalanced, euphoria when the mech makes a kill, etc, feel their arm go dead and numb when the mechs arm has been blown of, etc.
For the other style of control (sitting in a cockpit driving it with joysticks like a pilot) this would be cheaper/easier to produce, and suit non humanoid mechs well (armless chickenwalker mechs, quadmechs, etc).
Perhaps both styles should be in? Non humanoid mechs have the joystick style (perhaps with some level of neural interface, but a normal cockpit) and are generally used for certain types of roles (walking tanks, missile platforms, etc) while humanoid mechs move and fight like actual humanoid bipeds and are used for alot of the frontline combat, also being alot more agile, and better suited to having jumpjets/flying around, using a full interface where the pilot sees what the mech sees, feels what the mech feels, and the pilot controls the mech as if it were their own body, with no cockpit, instead more or less wearing the mech, all plugged in and etc?