I don't think that a 3D map (in the sense of having an extra dimension to annoy you and make things generally irritating) is a good idea. New regions (galaxies) make more sense and they play to what we've seen in recent science fiction - that is, the gulf between galaxies isn't all that interesting or inhabitable, but being able to get from one galaxy to another in a reasonable amount of time is pretty cool.
Skip to the List if you just want the Cliff's Notes...a manifesto is brewing here.
My dream is to take this concept a few steps farther. The germ of the idea came from a blurb for a mod where Earth nations still existed as separate entities. That got me thinking, that hey, since our petty conflicts and differing viewpoints were why humans were able to develop something as sci-fi basic as fusion reactors, then it would be more interesting if there was no Terran Alliance, but independent nations vying for their interstellar goals. Secondly, for me, the endgame of the large maps are just a bore. The matter of "galactic" domination was decided years ago (even by the standards of the wonky passage of time in GalCiv). Everything else is the inevitable denouement. Thirdly, with a tech victory equaling basically becoming an energy being ala the Ancients or the various god of the week in Trek, it's kinda sad an equal victory is taking over what's clearly not even an entire galaxy. Finally, just looking at the universe's arrangement: cluttered, then vastly empty, then cluttered, then empty (ie nucleus to atoms, sun to solar system), readily apparent that a loose repeat occurs and can be readily mapped. It's my dream that expansion in a 4x is nigh endless, but more reasonably that it extends far beyond even the immense settings currently available. I am well aware for fluff and crunch reasons, this will never happen with GalCiv, but it's a wishlist so might as well get it out there to be ignored
To lay it all out, I'd have 7 stages (I suppose if I really believed any of this had a shot, I'd cut out sections that are obviously untenable or just not that critical, but OCD compels me to list all of the ones in my head). Note, at NO point is it required to eliminate all opposition at one level to move to the next, ie your surviving rivals would also be present, though not necessarily capable of immediately joining you in expansion if you were the one to develop the techs. At present in my mind, all graduations are dependent on acquiring the tech needed expand farther. These will mainly be based around researching from one major grouping of propulsion techs to the next (ie Impulse to Warp, not Warp I to Warp II). A quantum leap represented by a huge payoff, and obviously increased research requirements, Star Trek does with with Quantum Slipstream, Transwarp Etc, or even simply STL to FTL. You can certainly ATTEMPT to fly an interplanetary level vehicle into deep space, but it wouldn't really get you anywhere...you'd likely lap it with your FTL ships a few decades (or months in GC) down the line.
1. Intercontinental, nations vying for scarce resources, possibly include orbit. Pretty much a low level Civ game. Develop an ion/impulse drive (not the Gal Civ version but the STL real world version) to move to next step, maybe the lowest version of lifesupport.
2. Interplanetary, Moon, Mars, perhaps a few Outer planet stations and moons. Develop hyperdrive, maybe the lowest level of sensors as well.
3. Interstellar, the standard GalCiv game it would seem. Plenty of stars, but clearly a defined region of the Galaxy, perhaps representing an arm of our spiral. Basically Star Trek speeds and scope. Minor races are now your rivals, though they'd obviously expand. Must develop some higher level of Lifesupport to make it to another spiral arm. Might need large hulls to pass anywhere near the probably more dangerous galactic core. Research to the next level of FTL, I guess it would be the "impulses".
4. Intragalactic, basically Star Wars level. Can handily jet around the galaxy now with varying times based on the level of Impulse. The main races now show up regularly now that you're traveling real distances as they've already carved out niches being much older and probably much more capable of creating fleets of colony ships even with such a radical new propulsion system. To get beyond this, your need the Warp propulsions to manage millions of lightyear distances and much more advanced lifesupport to even make it. You can manage this one of two ways. Either make THESE the gigantic+ maps, or start dividing the galaxies into districts and micro at that level. No way 1000's of worlds individually would even be fun, and that's even with my "OCD".
5. Intergalactic, basically SG level. Galaxies are the new solar systems. Can now travel to really far off places and make your own Shi'ar empire or be benevolent like the Asgard. Graduate with highest level of life support and Hyperwarp.
6. Intrauniversal, go anywhere. Local Groups are the new systems. Awe inspiring powers are represented here. I'm guessing the Dread Lords kinda fit here, though they never show a very impressive speed rating. Might also have fleet sizes like the special maps allowing for huge fleets.
7. Interdimensional (or timetravel, though that carries with it some serious dangers). Not really needed, but hey if we're wishing on a star...
Again, rivals carry over, though most of the time since it will be you going to another level, they will rapidly become obsolete, but they catch up and you could have a Cold War with the Klingons and say a NeoSov alliance vs the Federation and their North American/European counterparts. Idea kinda comes from Stargate where it was clear they were setting up the Chinese as a future threat of some sort, yet the Stargate program had dealt with galactic threats. Also raises the everpresent spectre of invasion from a species at a higher level than you that will require the entire map to join forces, if you can wrangle it. IE the "Dread Lords" special event exist at ALL the levels even if its actually just the Drengin for the first few ones. These seem reasonable as, to take an example from another sci-fi show, neither the Xindi nor Suliban are any type of threats to the Federation even by Kirk's time, but the Klingons clearly represent a "higher" threat that just happened not to care, and the humans enjoyed some minor amount of support with the Vulcans another "higher" threat.
As a gameplay thing, I would allow for planet templates like can be done in the shipyard for ships. That and mass selecting planets via the mouse. A semiautonomous building system might be combined with those. Kinda like the gambits of Final Fantasy XII such that automatic actions occur when a wide variety of conditions occur, and you can set priorities to each.
So it could be: build factories to set amount, focusing on tiles with manufacturing bonuses, then labs again focusing, and then whatever. Or make a factory template and churn out factories over and over again, stopping only to make a starport or something when you only have 2 free tiles on the planet.I wouldn't mind being able to mass select ships/fleets for movement also for that matter.
I would imagine this also opens itself to handling difficulty options, where your rivals start out far more powerful/with more resources than you or weaker depending on setting. The difference between being the Borg or the Tellarites...