It doesn't look like anyone's made a topic on shields yet, so I decided to put out a few ideas.
First of all, if the game is going to go for a mostly hard sci-fi background (excluding ftl drives/gravity generators) I think the ship's shields should be as close to real tech as possible. I found this on wikipedia and it seems to fit the bill fairly nicely: http://en.wikipedia.org/wiki/Plasma_window
. The page mentions that it could be used to keep air inside an open hangar. However, the plasma exists at 15000 K so this can only be used in airlocks where things with their own shields can pass through.
The wikipedia article on "Force field (fiction)" (under the Scientific Research section, so the non-fiction part) features a shield that envelopes a craft and protects it from radiation (lasers, perhaps) and "some particles" (that could include bullets in the game). These two technologies are quite similar so they could be combined as one shield generator within the game.
This shield generator would have an opening to the outside of the ship where plasma is constantly recycled (any recombined electrons/atoms are ionized again). This opening could collect all of the plasma particles to turn off the shield, as well as regulate the temperature of the shields (hotter shields mean better defences).
The ship would have to be plated with magnets to hold the shields in place so both installing a shield system and operating one would cost more money/energy for a ship with larger surface area (seems fair).
Damage reduction from the shields can vary depending on the weapon used. Plasma is translucent so some radiation must be able to get through, thus shields are not fully effective (maybe 90%) against lasers. On the other hand, bullets that strike the shield will be fully vaporized, unless they are very big, fast or well insulated (opening up to a wide variety of weapons and tactics) but will use some of the shield's energy to be stopped.
As for upgradability, going with the noble gases as the shield medium works out very well. Helium would be the starter shield material, as it's the cheapest/most abundant, but it costs the most energy to operate the shields as its ionization energy is the highest of the noble gases. As you go down the group the gases get easier to ionize but more expensive, so the increase in efficiency is balanced by the cost of upgrading. Each noble gas has a different hue when ionized (helium-pinkish, neon-orange/red, argon-purple, krypton-white and xenon-blue, radon gets left out because it's unstable) so upgraded shields give an aesthetic difference along with better performance.
There are a few other uses to the above shield system (google plasma stealth and plasma drag reduction, features which could be used at reduced shield power in-atmosphere) which can be looked into later in game development, but for now I think the above information is more than enough of a basis for ship shields.In short: Shields made of plasma protect the ship from radiation and particles (radiation penetrates slightly, particles weaken shield slightly) which can be upgraded for improved efficiency, at a cost.
Please leave suggestions for other shield designs/improvements on the above design. Thanks for reading.