To make it clear, in the situation above the builder must be positioned so that in one move it can become adjacent to the king, in order for it to be in check. No move other than Kd8 is possible in order to get out of check. Any attempt to block the builder won't work because:
a) if the rook tried to block the builder, it could still switch the black bishop for a white, checking the king, and
b) if the bishop moved at all, the builder would simply take the king, thus any attempt to move the bishop would be illegal.
This makes the builder way to powerful. An interesting variant would be to have an anti-king instead of a king.
Another thing I've been thinking about is what happens if a piece moves adjacent to a builder? Is that suicide or does nothing happen at all? The latter would make it impossible for a builder to capture another builder, is that good or is that bad?
a) if the rook tried to block the builder, it could still switch the black bishop for a white, checking the king, and
b) if the bishop moved at all, the builder would simply take the king, thus any attempt to move the bishop would be illegal.
This makes the builder way to powerful. An interesting variant would be to have an anti-king instead of a king.
Another thing I've been thinking about is what happens if a piece moves adjacent to a builder? Is that suicide or does nothing happen at all? The latter would make it impossible for a builder to capture another builder, is that good or is that bad?