[glttrljh] strictly weaker than queen

some fairy chess pieces whose movement is a proper subset of queen moves:

ferz
wazir
commoner (ferz + wazir compound)
bishop
bishop + wazir
rook
rook + ferz

underpromotion of a pawn to any of these will probably be rare.

design a chess variant in which underpromotions are more common.  e.g., you may not promote to a piece you already have.

design a chess variant in which underpromotions are permitted but always disadvantageous.  e.g., game scoring is not done by checkmate or stalemate, but by amount of firepower on the board.