Hexagonal grid
A game map divided into hexagons instead of squares, so all six neighbours sit at exactly the same distance from the centre.
Square grids have a built-in flaw: the diagonal neighbour is √2 times further away than the one next door, and every game has to choose between pretending otherwise and penalising diagonals. Hexagons only have edge neighbours, six of them, all equally far. That is why hexes became the default in board wargames by the 1960s, and why Civilization V switched from squares to hexes in 2010.
The bill arrives in the code. A hex map does not drop neatly into a 2D array, and odd and even rows shift differently depending on convention. What most people use now is cube coordinates — three axes that always sum to zero — which turns distance, rotation and line drawing into plain arithmetic. Amit Patel's write-up on Red Blob Games (2013) is the page nearly everyone who has implemented a hex grid since has kept open in a tab.