With bugs ( Bugs and Improvements - #1393 by SLKLS ) distorting the view on how the current versions of STL engines actually are I don’t really want to suggest how to dratically change them as we don’t even know what the current standard is.
But one thing could be to increase the fuel efficiency of STL engines (except FSE) to make STL travel better, while at the same time increasing time needed for FTL travel. This means that in system and short range trips become better, trips with a few jumps don’t change but long range travel becomes less worth it which incentivises local production and could crack up the current “just ship everything to the core” meta.
Another thing I would like to see would be in system FTL, this could for example be realized through a flight planner segment (with optional toggle like gates) that only scans if a FTL reactor is present and not if it has enough energy to move the ship to the next system.
So a “FTL Capacitor” that “can be charged with the energy of the normal power supply because in system distances are so small”. The ship would need BRS, FFC and field emitters + the “FTL Capacitor” to to use in system FTL, with that it would still be lighter and cheaper than a real FTL ship and can work more efficiently in STL.