I would keep the current races that are in the game and add probably 4-6 others including a nomad or pirate race which could be played by the AI as well as a human player.
Increase the types of maps and include a random factor of a 2nd hidden, (smaller) universe which you would find hidden somewhere within the map. This univers would be smaller then the original (Due to space limitations) but would include say up to 50 planets & moons, maybe have 2 or 3 hidden entrances and exits that have to be found
In corporate a number of ideas from the various current mods like various weapon mounts, I also like the idea of keeping anti ftr and anti seeker weapons seperate so one does not shoot at both
add new additional types of ships, like corvette, super DN, monitor etc, just like in some of the mods
Would put in a better ground combat system, possibly based on something similiar like that of the old Star General Game (simple but fun) you could chose ground combat say anywhere from 5-10 ground combat rounds per 1 space turn. This would be actual combat between the differnt types of units, you could have infantry, aritllery, planes, tanks, etc.
Capability of adding a monster race or two if one wanted to
Making the AI more affective and smarter (I know this is the hardest part

) but some of the mods out there for seiv are pretty good with some of the races
add more types of ftrs & seekers, give some races some type of bonus for their ftrs or seekers as this could be a specialty. Maybe make them harder to hit, faster, more accurate, maybe start out with a higher experience level or something on that level
just some ideas Mac