GC3 and the TACTICAL AND STRATEGIC ASPECT!
This has probably been said before, but I will say it again, since I feel it is important for GC3. Having all 'pure open space' is probably the most realistic, but takes almost any type of Strategic Play out of the game completely. (Make it an option during game setup... Amount of crap in galaxy: Dense, Full, Average, Sparse, Empty).
Just as example (not comparing), take Civilization (or many other types of those games), the main element of strategy came from the terrain. Mountains provided excellent defense, as did hills, rivers gave defense when defending on the other side, terrain made chokepoints and oceans divided territory, making it tougher to invade from the other side. These things often made the difference between VICTORY and DEFEAT!
I loved looking over the map trying to figure out the best way to overcome a superior enemy, or to defeat an opposing force the best way possible... because there were so many ways to do it! Civ3's WW2 Global mod made by Rocoteh and many others (myself included in some aspects) was a masterpiece of history which really made you use terrain to it's fullest ability to gain advantages.
Now these things can't directly apply to GalCiv (for obvious reasons), but many of their ideas can be implemented. Developing different kinds of space 'terrain' would not be too difficult, as there are many 'exotic' theories abound about various types of voids, matters, and all types of stuff that could be used.
You could have Interplanetary Space, Heliopause, Interstellar Medium, Interstellar Space, Subspace Corriders, Antimatter Clouds, Nebula, Clusters, Nova Remnants, Dust Clouds, Intergalactic Voids, Dense Asteroid Fields (other than the mining ones), Star System Belts, Solar Winds, Star Forming Regions, Exotic Matter Fields, Hydrogen Clouds, Energetic and Large-Scale Galactic Center, Filamentary Structures, Spiraling Magnetic Fields, High-Radiation Areas, Galactic Walls, Arcs and Space Threads, Paranormal Time Fabrics, Cataclysmic Variables, Gas Streams, Ionized Clusters, Lyman Alpha-Blobs, Dark Energy and Dark Matter Regions, Fractures in Space, JUST TO NAME A FEW! There's almost an endless variety!!!!!!!!
It wouldn't make the map some crazed messy looking thing, If graphics are done right, you would hardly notice the differences, they would be subtle, yet they would be there!
EXAMPLES:
1. Solar Winds would be like rivers, allowing you to travel slightly faster when in their path and more slowly into them.
2. Filamentary Structures would act like 'defensive barriers' giving ships on the other side a defensive bonus if an attacker has to cross the filament.
3. Voids would give increased sensor range since they are completely empty. (Voids differ from Space in that Voids are perfect vacuums).
4. Nebula would be incredibly dangerous regions to enter and likely to damage/destroy ships (would be good to create game chokepoints).
5. Asteroid fields and Star System Belts (like the Kupier Belt) give defensive bonus to ships. The list could go on and on.
IM TELLING YOU! Much better than making every ship, and having every ship fly through nothing until it reaches it's destination in nothing... this would create the TACTICAL AND STRATEGIC ASPECT to the game, that it is currently missing!