1 Planetary resource acquisition and factional economy (this usually comes hand in hand with other items on this list, so I assign it to the same priority)
2 Further large object optimization, improvement of effective logic update distance (while optimization is always good, as long as the game is optimized enough to be played, mechanics and content are worth more than performance)
1.5 Integration of RP/interior/crew into meaningful game mechanics (parts of this can come hand in hand with other items on this list, and as such, potential should be analyzed while tackling other mechanics, but it shouldn't be addressed directly before other mechanics are settled)
1 Additional astronaut mode content, including equipment (usually goes hand in hand with the above)
3 Graphical weapons update: better scaling of visual FX, explosions, smoke, particles, "battle flair" (graphics are not related to the game's mechanics and engine, and as such, look should only be improved when the rest is solid)
1 Mechanical weapons and systems update: Improvement and/or niche-finding for underused weapon/system combinations (a variety of actually useful weapons is always good, an underused weapon always needs care, unless it is redundant as another weapon combination can emulate it)
1 Single player content for both servers and actual localhost (mobs, NPCs, missions, trade) (usually also goes hand in hand with other points assigned to 1)
1 Fleet control (of varying depth; at least some basic commands like follow, protect and similar) (usually comes hand in hand with singleplayer content, as on multiplayer players form their own fleets and thus such controls aren't as necessary there)