There are several large mods on the way, some of which address this issue. For instance, I'm working on a mod based on the techs, and wonders of the Alpha Centauri game, although most of the units come from other SciFi (like
Starship Troopers and such. Currently I'm at 45 techs, a few dozen wonders and buildings, ten new social policies, and twenty-some-odd units. It's ALMOST ready to be tested, and I'll make a thread on this board once version 0.1 is ready to go (although I have no artwork yet).
1. Can new units be added and if so how would I go about creating new skins for them? I was thinking I'd just use existing ones as a template and go from there
New units can be added easily, statwise, although "special" abiltiies will be limited to basically what already exists. There are a few stubs in place for some abilities that aren't currently being used (HiddenNationality, for privateer-style units, and you can limit special "hero" units to one per empire just like national wonders), but for the most part you're limited to what's already being used. Also, while many of those stubs might be in the .xml files, there's no guarantee the code is actually functional in the game itself. (For instance, Religion is mentioned in quite a few XML tables, and adding new strategic resources requires a bit more than the XML implies.)
But that still leaves you a lot of leeway. For instance, the top unit in my mod, the Gravship (think Starcraft's battlecruiser/carrier mixed together) is a 200-attack, 200-ranged-attack unit that can carry 4 fighters, move across any tile for 1 MP (including water and mountains), and has a half-dozen other bonuses and penalties. But you can only have one of them per civilization, and by the time you get to those, the other victory conditions are all easily achievable.
In terms of graphics, that takes a lot more work, and I haven't even tried to start doing this yet, although supposedly it's not bad if you've got a 3D modeling program already. So for now I'm using existing units as placeholders for everything; that Gravship uses a Battleship graphic, the Assault Powersuits look like roman legions, and so on.
2. The same question but applied to tile improvements/buildings. I want to actually update tiles to look more futuristic
Same basic answer. Easy to stat up new buildings (although again, you're mostly limited to the sorts of effects other buildings already provide), hard to do the graphics for them. There's a bit more flexibility to work with than for units, though. For instance, in Kael's guide there's a good example of him using one of the three unit models in the standard Barbarian Brute unit to act as the basis for his new warrior.
I'm drawing my ideas for units/techs/buildings/research from a couple of science fiction series, first the Axis of Time trilogy by John Birmingham, and second the Legacy of the Aldenata series by John Ringo.
Not sure how well the Aldenata books would work. By the third book, there's really only a handful of human units mentioned as being effective at all (powersuits, artillery, stealth non-combat shuttles, SheVas), with the more mundane stuff all being wiped out early in the war. Not counting the space battles, of course. The other "good" races didn't fight (except for Himmit spies), and the Posleen only had three unit types at all. But if you're just mining them for ideas for a few unit types, it's okay.
The biggest problem with a Future mod is a simple one: making sure the game gets TO the future eras. The core game is easily winnable in the Industrial or early Modern eras. There's not much point in adding dozens of new techs if you never make it that far (outside of specific scenarios or games that start in the Modern era). So, in my mod I had to add a bunch of things designed to slow down the other victory conditions:
> All units get +10% to combat when in their own territory. All units get an ADDITIONAL +10% when ATTACKING in their own territory. (Combined, these make Domination a lot harder.)
> Cities now have more defense and restore their health faster. (Again, harder Domination.)
> Cities take a bit more food to grow. To compensate there are more buildings that provide FoodStorage bonuses (a la the Hospital), but getting them all will take more construction time than under the old method. So cities won't get big quite as quickly, which slows down the late-game tech race a bit.
> Building the spaceship no longer wins the Science victory. For that, you need to build the Transcendence wonder much later in the game. The spaceship now gives you other benefits instead. (Free SPs, free techs, long golden age, etc.)
> Each social policy branch now has six SPs instead of five (meaning you need 30 to win a cultural victory instead of 25), and the sixth in each tree doesn't unlock until an appropriate tech in one of the future eras so it's utterly impossible to win cultural before then.
> Diplomatic victories now require 80% instead of 66%. Unless you've gone on a massive conquering spree, this effectively requires ALL city-states to be on your side.
That sort of thing. There are just a lot of little things you need to watch out for.
