Source: http://cargocollective.com/annapurna/Illuminatus You can do them in different colours and go crazy paving your house
Visual appeal: Natural disasters: Tornado, Volcano, Earthquake, Tsunami etc Interactive appeal: This is a untapped area with bullion coins. Something which people can use while admiring the design of the coin. Chess pieces: Rook, Knight, King etc Wheres wally coin that come with a miniature magnifying glass?
OK on a serious note!! Video games, characters from popular games, Billions of people play these games and collect everything. There is a ton of art for Battlefield 4.
Don't be afraid of sharing unless you are dead set and dedicated on actually making your own series. Its hard works and costs thousands of dollars. You have to locate a mint that will do the work, spend BIG money on making a die and most places have very high minimum mintage numbers. Not easy.
Ok. I'll tell y'all my second best idea. Get the backing of a zoo and mint coins of the animals. Profits go to the animals upkeep.
What about a commonwealth them. Australian, Canadian, NZ, UK... and have HM on the obverse. Wait!!! Victoria Cross winners, these guys are hero's and i would buy a dozen.
Lost civilizations (Aztecs, Incas, Mayans, etc). or Mythical cities (Atlantis, Cant think of another but sure there are more cities). or Dinosaurs (Its been done before, but still a cool subject)