I think the current Mech Efficiency System is quite boring. Every mech has the same efficiencies, and when you switch to a different mech, you have to take them again.
Short Version:
So, my idea is - let's create more mech specific efficiencies. There will still be general efficiencies that are shared between all mechs, and some efficiencies will be "repeated" between different mechs. Some are very specific -for example, the Hunchback may get a special module that is devoted to its "Hunch", the Centurion may get one for his "Shield Arm", and so on.
The General Mech Efficiences change a bit - instead of having to "relearn" for each mech variant, when you change to a different variant of the same chassis, you keep the old efficiency, but you can also spend additional XP to raise the Efficiency. The final cost and bonus would still be the same as now, but you get to keep it around.
Once you have unlocked the first rank of all Basic Efficiencies, you can unlock special "Mech Efficiency" Modules for the Mech. These modules go into a "Efficiency Module" slot on a mech, which are in addition to existing module slots, but slot only different efficiency modules, not pilot modules. This means you cannot run all the modules you acquired together - you must select a few of the modules in accordance to your personal preference. This encourages players to keep tinkering with their mech modules even after they acquired them all.
Long Version:
General Efficiencies
Basic Efficiencies
Each Basic Efficiency comes in 4 ranks. Each rank costs 1/3 the current XP value, and grants 1/2 the current bonus. The Basic Efficiencies apply to each variant of the same mech, but you must earn and spend XP on different variants to improve to the next rank. (So if you want the bonus of 4 ranks, you need 4 variants, but when you start training on your 3rd mech variant, you get 2 ranks from the previous 2 mechs variants from the same chassis.)
There is no more "double bonus" when you mastered all Basic Efficiencies. This is replaced by the ability to go up to 4 ranks, meaning you can reach double the current bonus.
Elite Efficiencies
These work identical to Basic Efficiencies, you get 4 ranks for each efficiency, each costing 1/3 the current XP for the skill and giving 1/2 the bonus.
There is no more "double bonus" when you mastered all Basic Efficiencies. This is replaced by the ability to go up to 4 ranks, meaning you can reach double the current bonus.
Master Efficiencies
There are now two Module Slots you can learn here - one for Pilot Modules, and one for Efficiency Modules. You still learn this efficiency for each mech individually.
Efficiency Modules
Weight Class Modules
Requirement: To be able to spend XP on Weight Class Modules, you need to have reached one rank on all basic efficiencies for at least 2 different chassis of that weight class.
Chassis Modules
Requirement: To be able to spend XP on Chassis Modules, you must have reached 1 rank in of 3 variants of that chassis.
Chassis Variant Modules
Requirement: To be able to spend XP on Chassis Variant Modules, you must have reached 1 rank in all elite efficiencies of that chassis.
Efficiency Module List
Spoiler
Light Mech Class Modules
Agility: +10 % acceleration and decceleration bonus, and +10 % turn speed
Reinforced Legs: +20 % internal structure for legs
Medium Mech Modules
Stability: +20 % faster standing up animation, and suffers 25 % less effect from impulse weapons.
Rapid Fire: +10 % rate of fire on all weapons.
Heavy Mech Modules
Endurance: +10 % heat capacity
Large and in Charge: +10 % speed and +10 % acceleration and decceleration
Assault Mech Modules
Robustness: +10 % internal structure for all torso sections, and internal components take 10 % less damage from attacks.
Sustainable Firepower: +10 % heat dissipation
Commando
Overgunned: All weapons gain 20 % range
Robustness: +10 % internal structure for all torso sections, and internal components take 10 % less damage from attacks.
COM-1B:
Short Pulsed Lasers: Beam Duration of all Lasers reduced by 25 %
Com-1D:
Endurance: +10 % heat capacity
COM-2D:
Rapid Reloading: +10 % recycle time for ballistics and missiles.
COM 3A:
Sutainable Firepower: 10 % heat dissipation
Raven
Awareness: +25 % Radar Range
Stealth: Enemies detect Raven at 25 % less Range
Raven 2X:
Short Pulsed Lasers: Beam Duration of all Lasers reduced by 25 %.
CDA-3C
Increased Storage: All Ammo is increased by 25 %.
CDA-3M
Rapid Reloading: +10 % recycle time for ballistics and missiles.
Hunchback
Robustness: +10 % internal structure for all torso sections, and internal components take 10 % less damage from attacks.
Leg Reeinforcements: +20 % internal structure for legs
HBK-4G, HBK-4P, HBK-4J:
The Hunch: +20 % internal structure for the hunch, and +20 % damage reduction for all internal components installed there
HBK-4SP:
Safe Storage: All mech torso components are treated as if having CASE installed.
Centurion
The Shield: The melee weapon arm has +25 % internal armour
Survivor: All damage from critical hits is reduced by 25 %.
CN9-A:
CN-AH:
Rapid Reloading: +10 % recycle time for ballistics and missiles.
CN9-AL:
Energetic Efficiency: 7.5 % Cooldown and Heat Reduction for Energy Weapons
CN-D:
Safe Storage: All mech torso components are treated as if having CASE installed.
CN9-YLW:
Increased Storage: All Ammo is increased by 25 %.
Dragon
Agility: +10 % acceleration and decceleration, and +10 % turn speed.
Stability: +20 % animation speed against knockdowns, and 20 % more resistance to knockdown effects