What are your interests and her now.
Packing a 300 gal with goldfish would be great. What would be their adult size.......
A 300 gal discus tank is lovely for carefully chosen colours.
a 300gal blue zaire adult gibberosa community will be stunning.
300 gal marine with only Queen Angels......
A bigger sized tank with a "hump headed wrasse" try this its for you......