-Generator variant, called "Matrix" ?
-Peces on first rank never move but generate new pieces. Pawn don't generate.
-A generator captured and replaced by a similar piece does not generate.
-King does not generate. No castling. En passant possible?
-Goal is still checkmate.
+"FreeBoard", re-using a lot of Board logic, but with SVG (empty) board + SVG (empty) reserves.
+Will be used for variants with custom non-rectangular board (Hex, at least)
+Or, with other board shapes (see greenchess.net for example)