Crew modifiers would be interesting. I know that Jay's VBAM/SX book touched on that for purposes of VBAM conversions, but I have been tempted to sit down and try to match up the Companion officer abilities to SX to get an idea for how that would work. A dedicated system of rules specifically for SX would be a nice addition, though.
A "black book" supplement that included expanded technologies in areas wehre it is viable would be nice. As Cartman mentioned, unreliable/weakened technologies would be a nice addition, though mostly I see it being beneficial for campaigns. Similarly, looking at the systems I can see generational PDS, drones, and mines being a possibility. Again, their utility is limited to campaign play, so as a stand alone supplement is probably would not constitute enough material.
I think the one avenue that SX has for new releases is likely in the form of universe books and ship lists. Put together a nice, full supplement with background, ships, and scenarios and you would probably have it made.
I have talked to Dan about doing a SX crossover for the Escalation Wars stuff, particularly getting things ready so that there could be a SX ship book release at the same time the VBAM war history is released. In a book like that, you would be looking at a good 250+ ships, more than likely, covering a three-hundred year period in the setting.
The only other thing I would have on a wish list is an even more advanced, detailed method of designing fighters. Setup a system where fighters are considered a weapon system maybe, so that they could potentially have variable Movement Range, ROF, PEN, DMG, and maybe some special effects. The basic and custom fighter rules are nice, but I am still left wanting something a bit more meaty so that fighters can be more variable in abilities.
-Tyrel