Well, I don't want the player to always choose the strongest piece plus a bonus. By only offering a piece that otherwise would never be chosen as part of a pair, it can be made a necessary choice. FD already seems very strong for a bonus piece. If that option was available for any Pawn promotion, it might pay to do 'suicide promotions', where you promote unprotected in the certainty that you can keep one of the pair. If the bonus piece was of Pawn value, you would gain nothing that way. This brings me back to fFbW.
[Edit] The fFbWbD might be a good choice for a bonus piece. Because it attacks orthogonally adjacent squares it can in principle deliver checkmate, but with only 4 move targets it is way too weak to do that on its own. It can do it together with all other Team-Mate peaces, though, even the very weak Mortar. In a reasonable number of moves. And it is not color bound. It can even force checkmate together with its mirror image fWfDbF.
So I guess a good solution would be to offer promotion to this mirror-image pair, in the orientation that they do not protect each other. (Which would be a different orientation after a capture than after a non-capture. The pair then should not be worth more than the other pieces, as the value of these pieces should be similar to that of a Ferz (1.5 Pawn). The KPK end-game could still be won through such a promotion. And piece + pawn vs piece could be won if you manage to get the pawn to the (protected) promotion square, as the defending piece then could sacrifice itself for at most one member of the pair.
Well, I don't want the player to always choose the strongest piece plus a bonus. By only offering a piece that otherwise would never be chosen as part of a pair, it can be made a necessary choice. FD already seems very strong for a bonus piece. If that option was available for any Pawn promotion, it might pay to do 'suicide promotions', where you promote unprotected in the certainty that you can keep one of the pair. If the bonus piece was of Pawn value, you would gain nothing that way. This brings me back to fFbW.
[Edit] The fFbWbD might be a good choice for a bonus piece. Because it attacks orthogonally adjacent squares it can in principle deliver checkmate, but with only 4 move targets it is way too weak to do that on its own. It can do it together with all other Team-Mate peaces, though, even the very weak Mortar. In a reasonable number of moves. And it is not color bound. It can even force checkmate together with its mirror image fWfDbF.
So I guess a good solution would be to offer promotion to this mirror-image pair, in the orientation that they do not protect each other. (Which would be a different orientation after a capture than after a non-capture. The pair then should not be worth more than the other pieces, as the value of these pieces should be similar to that of a Ferz (1.5 Pawn). The KPK end-game could still be won through such a promotion. And piece + pawn vs piece could be won if you manage to get the pawn to the (protected) promotion square, as the defending piece then could sacrifice itself for at most one member of the pair.