- cool-down
- heat generation
- range
- accuracy (velocity for projectiles, spread for missiles, and duration for lasers)
It could be 4 smaller trees, or 2 big ones (lets say 1 tree for range and accuracy and another for the rest), nodes in those can affect 1 or more weapon type in 1 or more way, structure of the tree itself could be done in any way, but as a whole it will represent one-two aspects of every weapon in game, so lets say maxing cool-down tree would get cool-down reduction for whole weapon specter. Of course some weapon-specific nodes could be mixed-in too, so maxing whole tree for just 1 weapon wont be necessary.
I think skill trees build that way would provide more flexibility in builds, wont encourage boating as much, and focus on role specialization more than on weapon type (like brawlers can get cool-down and heat gen nodes and ignore range and accuracy). Of course getting most from every tree should be possible too, but with trade off of points which would go in another tree otherwise.
P.S. well this is just a concept i thought of after i saw a ton of trees for each weapon type, from whose only one would be fully maxed depending on what weapon is main on current build, and others wont be even touched. I think this concept at least more interesting.