The current boosters we have in-game are honestly boring, since they both do the same thing, except that Overdrive can be used an unlimited amount of times, and Accumulator doesn’t generate any heat. They’re both good in terms of playstyles, but I think they would benefit with having an upgrade path.
Each booster can support up to 3 upgrade levels, and you can change the “power level” of each booster from stock to level 3.
Overdrive
Upgrade 1: 400 keys, increases boost from 50% to 55%, weapon overheats 55% faster instead of 50%
Upgrade 2: 500 keys, increases boost from 55% to 65%, weapon overheats 65% faster instead of 55%
Upgrade 3: 600 keys, increases boost from 65% to 75%, weapon overheats 75% faster instead of 55%
Handicap: Your weapon will overheat faster with each upgrade level.
Accumulator
Upgrade 1: 300 keys, 20% more accumulator capacity, charges up 5% slower
Upgrade 2: 500 keys, 35% more accumulator capacity, charges up 10% slower
Upgrade 3: 700 keys, 50% more accumulator capacity, charges up 15% slower
Handicap: Booster will charge up slower.
Edit: I realized this sounds like another key sink where people would not want to have the base version so I changed a few things. Thank you @Sammarald!