Relativity in this would be awesome. I found it kinda annoying when I decided spawn earth around a 1 light yr diam star to find earth going at 8c.
You could just have the planets increase thier mass as they get past 0.9c (diam must stay the same). That would make it much more realisitic.
Black holes need to destroy/freeze what get close to them as well as a side note.
As for other properties
Length - I dont know if the engine supports objects changing shape. Anyway, you would have to calculate what velocity the camera is going and change the length accordingly which would be hard.
Light - C is constant everywhere. Same problem with length, you would have to calculate camera velocity. Even then if you calculated that, I dont know how you could represent the light properly without making the game unplayable.
But mass changes would be awesome and I would imagine not too hard. At least not hard compared to the above two.