There is a question about the two essential elements of chess variants.
In some chekers variants a cheker can promote into a king, then we get pieces with different powers. This is one of two most widespread and important similarities. But chekers is still not some chess variant. We can even desighn a chekers variants with king in initial setup. Is this a chess variant?
Is the breakthrough game Camelot a chessvariant? It has pieces with different powers.
An elimination game with pieces of all pawns (or rooks, or queens) and a chess board is also called a chess variant. The reason is probably that pawn is derived from chess.
Once again, are these two most widespread and important similarities essential to chess variants?