First, the stuff that, while SD has done a great job with the entire concept of GC2, really should have been done long ago...
1. Bug fixes - hate to point it out, because admitedly SD support is mostly soooo good, but plenty reported that nothing has seemingly been done about in literally years?
2. Corrected and consistent descriptions of race characeristics, techs, weapons, buildings and effects.
3. Corrected and consistent displays of costs, incomes, etc.
4. The interaction of the three sliders (mil-soc-rsrch), when the numbers update from another slider moving also update the slider positions, you know, so you do not have a 50% slider on the far right etc. Just an annoyance but c'mon for goodness sake it ain't that difficult to code.
Then, okay, new stuff...
5. In general, add numeric entry options next to all the sliders and link them two-way to the slider positions.
6. Change galaxy setup nomenclature ie 'abundant' means how many, etc?
7. A relatively easy one to code - a new check box at game start (galaxy set-up screen) to have everyone start the game with no ships pre-built... no miner, no colony, and definately no flagship. Let the players get the techs first then build them themselves, or not. YOu could even extend this to adding rudimentary colonization and space mining techs to enable those modules but that would be essentially another seperate idea altogether.
8. Another easy one, easy to code, not easy to play - a new check box at game start (galaxy set-up screen) to have everyone start the game with no cash on hand (yeah, I know, tough), or better yet a numeric entry for how much.
9. On the combat movies, make the beam shots multiple shots that last as long as the missle and gun shots do, you could make this a selectable option (aside from also fixing the bug that the missile and gun shots cause leaving the ships circling forever).
10. On the combat resolution screen for each shot, show the attacking weapon type (cool name) and defense (cool name) used together with the factors and the math applied (show bonuses etc), so you can go back over it after the battle. If some, even most, folks do not really want this then you could make this a selectable option on the game play interface or video options screen.
11. More animated animations, with more movement. You can barely tell wether the negotiators are happy or angry.
12. 'Phasors', 'Photon Torpedos' ... c'mon guys, put some creativity into this instead of copying stuff made up in some 1960s low-budget TV series no matter how good (although keep the toungue-in-cheek comedy robot and the attitude to go with it, just be original - and yes I know phasors are a mathematical term but I think the derivation here is quite clear)
13. Finally, my pet idea and a bit more involved, a new check box at game start (galaxy set-up screen) to have everyone's ship damage as a percentage of hit points affect all the ships characteristics (attack, defense, speed, sensors, range, troop/colonist capacity), during battle so it affects even ship targeting in the middle of a combat round (meaning also damage will get spread around similar ships not just piled on one ship).
Issue I see in #13 above is with the defender shoot-backs, so maybe only take effect when hit points actually take effect at end of each combat round, but have the combat resolover on both sides use the next round scores, ie adjusted for any hits, to determine targeting - although thinking about it I believe thecombat resolver already uses next-round scores for target selection since it accounts for ships being destroyed and targeting the next one even though the destroyed one can still shoot back. One (desirable to me) side effect of my suggestion would then be that even damaged ships would cause the target selector to find the next similar undamaged ship in the fleet, rather than the current pile-on (sometimes-over-)kill.
I also really like the planet movement about stars idea, but I would never expect to see it - that would just be waay waay too big of a change and break probably all the code in the game and AI and require a whole new series of betas to test (heck, to read the posts there is apparently still an issue from years ago with fleet autopilot so I can not see fixing autopilot let alone all the rest of the stuff to work with moving planets). But an elegant, very nice, idea anyway.