Or the big fish on the reverse swim round the coin eating the small fish, every year the whale and dolphins move around the coin clockwise and devour all the small fish until the final year where there is just a big ol whale on the reverse.
Or, pay homage to all the great govenment bullion coins out there, have the whale strike a pose similar to libertad (jumping with the mountains in the background) - britannia (jumping out the ocean and the flowing dress of britannia is the waves splashing up) have the steem logo on the reverse look like a privy mark, a maple shaped ocean with the fish swimming round etc..
Or, this type of thing is quite a simple and striking image;
And you could make one part of the design an actual star constilation and change the constilation every year, so start with Aquarius or Pisces (fish obviously):
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