Gas giants should be like stars in that they should have a huge grav wells (intentional for ships to take longer to jump in/out). Plus it could have a small colonizable moon (use asteroid model as placeholder for now i guess), and have 1 or 2 crystal roids. That way it wouldnt be totally useless. However you can only build in a small area around the moon you colonized.
Id like to see different types of planets, and moons in game, or 2 colonizable worlds in the same grav well (planet, and small moon). Of course these things would have to be balanced out as to not screw up gameplay.
personally I think as P5y suggested by keeping the neutral, or pirated owned, but not any of the races / factions, but then again gameplay is important, so there could be other uses for them, but yes I agree, other planet types and moons would be cool to see