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Here is a light-hearted idea relating to planetary dynamics that you might want to add: Orrery Chess. A physical board would take quite some construction but it should be no more difficult to simulate on a computer than static-board variants. It has a circular board with 4n cells on the nth ring out from the centre, for n from 1 to 8. Between one player's move and the other's, everything moves round one cell anticlockwise, which is a right angle on the innermost ring but only 45° on the next out, 30° on the next, et cetera. If you are interested I can suggest a possible array and moves, which are necessarily quite a departure from square-board moves.