I donít think it would be a good idea to get another goldfish. Most people would say that the minimum tank size for a goldfish is 30 gallons. Maybe a school of other fish would be better. Neon Tetras or GloFish (Danio or tetra) would look great in there. You could keep about 15ish neons and maybe about 12ish GloFish.
Everybody is a genius. But if you judge a fish by its ability to climb a tree, it will live its whole life believing that it is stupid.