As far as supplements go, I would like to see a complete rules rewrite that clears up some common rules questions as well as adds some rules modifications.
Most importantly though, ship and aircraft conversion data has been requested in multiple posts/threads, and in several cases there was consideration of possibly posting the formula in S&P. I would be just as happy if this was included in a rules re-print as suggested above.
If the playing universe had the conversion data, think about the scenarios and ship data that could then come from the general gaming populace based on their interests. For instance if you wanted to put together a 'Gator' fleet, you could convert data for LST's, LSD's, APA's, etc. This could apply to other minor navies as well, or even ships that were not charted for the major navies.