What about only allowing superpawns to promote to knights or bishops? KR vs. K is as much a win as KQ vs. K, and being a rook up in most other positions is pretty decisive. But KN vs. K is a draw, and being a knight up is less decisive. On the other hand, I'm not sure the benefits of a super pawn are worth quite that much.
It gives me another idea for a variant, though. What if a pawn in enemy territory (the far four ranks) can move backward. That is, it can move straight backward without capture, or capture diagonally backward. That wouldn't help them promote, but would make advanced pawns much more dangerous (as a pawn chain can support it's own tail).
As always, the proof is in the pudding. Have you tried playing this with your dad? How'd it go?
This variant (don't have a name for it, but I guess it can be called Superpawn Chess) is almost the same as normal chess, but includes a new piece called the superpawn. Before the start of the game, the white player chooses two of his/her pawns to turn into superpawns. Then the black player does the same. It can move straight forward, but not to capture. It can move and capture forward-diagonally and sideways. It cannot move backwards. The superpawn can, just like the other pawns, move two squares straight forward when it is in its starting position [edit: and has not moved]. The superpawn may not take en passant, but can be taken en passant. In order to keep it from being too strong in the endgame, it can only promote to a knight, bishop, or rook, but not to a queen. It looks the same as a normal pawn, but with two lines through it so it is easier to tell it apart. In OTB play, you would either remember which pawns are superpawns or put a checkers under them. All rules not involving superpawns are the same as those of standard chess.
Anybody think anything should be changed in this variant? Originally my dad made up the idea of the center pawns always starting out as superpawns, but then we decided that this way would be more fun. Also black getting to counter against where white puts the superpawns makes up for white's first move advantage. Thoughts?