A docking port. To allow ships to dock in space and transfer cargo, fuel, ammo*, and hopefully someday personnel. It would also permit large ships to dock externally at outposts.
As for downsides beyond occupying a module slot: Perhaps if it were destroyed it would vent atmosphere the same way a broken canopy does. Outposts could charge you a handsome fee for docking a large ship, to ensure medium ships still had an edge in profitability when trading at outposts. The fee could be justified by saying the stresses of docking large ships involved high maintenance costs for the outpost, or maybe even that altering the outpost's centre of mass with a large ship required performing costly station-keeping/stabilisation burns.
*Which would be storable in cargo racks in arbitrary quantities in this fantasy.
Yes to tractor beams!!
Now that we have the Thagoids to pal around with, I guess it would be possible to reverse engineer one of their tractor beams? Although I doubt the technology is compatible... yet.
High Wake Hyperspace tether... Pulls you through hyperspace with your target... with the cost of fuel etc being on the pulling ship... and dumps both ships in to normal space on the other side.
Downside... Shuts down your Frame Shift Drive so you have to reboot it...
Supercruise booster.
Pro's:
Extreme speeds in supercruise (I'm thinking Hutton in 15 minutes if you could use it constantly for 15 minutes; which you can't, but it does give you some idea of the speeds I'm thinking of)
Cannot be interdicted
Can be engineered
Con's
Lasts for a certain period of time (2 minutes for example)
Requires a recharge directly proportional to the amount of time used
Cannot steer
Universal limpet controller. The limpets can be any one function you want at a time.