The light frame is limited on it's reactor size, max shields and armor. Heavy has large reactors and heavy shields/armor but has less mobility.
Medium lets you blend the two, with a decent amount of power and mobility. It's the most flexible of the frames but doesn't max out on speed like light, or power like Heavy.
Thanks for the reply
@Grummz
My biggest concern here is uniqueness. The Medium sounds like a jack of all trades sort of frame, not really bad at anything, not really good at anything? Besides "dual wielding secondaries" how do you plan to uniquify the medium frame.
Perhaps we need another major stat besides just mobility and power so there can be a balanced triangle instead of just 3 frames on a gradient? I remember 0.7 Firefall had a fairly decent triangle based on Mass, Power and CPU's. If Mass equates to mobility then can we get a CPU version for Em-8er?
EDIT: What if each frame has a unique "Core", Light frame has a Hydraulic core, Medium frame has a Processing Core and heavy has a power core.
Hydraulic systems benefit mobility and abilities that augment speed and maneuverability. The light omniframe gets a base +X% movement boost and the hydraulic core gives a bonus to any ability utilizing hydraulic systems.
Processing systems benefit offense, abilities that directly damage or augment the players ability to attack their enemy has high processing requirements. Medium Omniframes get a +X% damage boost and the processing core gives a bonus to any ability utilizing processing systems.
Power systems are needed for energy intensive defense and abilities that augment such. Heavy omniframes get a default +X% shield boost and/or some damage mitigation, additionally a Power core gives a bonus to any ability utilizing power systems.
As in the above diagram there would definitely be some overlap between the frames and even a fair number of abilities that all 3 can equip but this would provide a natural limitation as well as make each frame more unique in its playstyle.
Basic Example:
Cannonball - the omniframe leaps forward and smashes into the ground, damaging enemies in an area around it.
2x Proccessing System, 3x Hydraulic system.
Light Omniframes get a 30% bonus on the movement aspect of the ability, allowing them to leap further and faster than the other frames.
Medium omniframes get a 20% damage bonus to the smashing aspect of Cannonball.
Heavy omniframes can equip the ability but gets no bonuses as it is not a defensive ability.