But your own original upgrades, like fire or heat damage, physical and energy.
The decision is no limitation and doesn't upgrade unless you click the weapon tier 2.
Weapon tier 2 allows you to have a higher damage upgrade capacity.
Also make it possible for the flexible upgrades to have applyable attachments too when upgrading them through garage. Like an Rocket launcher added to your mech hall and fires during end phase like the spawn bot.
Passive weapons? Maybe a weapon you can't use but shoots anyway. The basic light minigun maybe.
Freeze weapons / Coolers on weapons
Ice damage is heat damage negative making the target or your heat go down when using it.
Something like the old item usage boost only as a weapon.