Hey, welcome to the forum!
The only traits I currently have plans to add are ones that affect ship systems (cooldown, flux cost, or recharge rate - whichever is most applicable). That will have to wait until the next release of Starsector though, as it doesn't fully support what I want to do yet. It might not work out though, as complications with something like that are likely.
I try to avoid adding traits that have too much overlap with each other, so I think +/-fire-rate would be too similar to +/-damage, and +/-flux-cost might be too similar to +/-flux-dissipation. No traits currently affect projectile speed though, so there will probably be a trait that affects that some day.
I've been considering adding traits that only affect specific weapon types (ammo recharge rate? energy weapon flux cost? beam damage? ballistics projectile speed?), but I'm not sure it's a good idea yet. Missiles already get special treatment, so it only seems fair...
Another idea I liked that someone brought up a while ago is a trait that provides some sort of bonus that increases with the number of d-mods a ship has, but I haven't ironed out the details for making that work well yet.