'pawns can only be dropped on the penultimate rank' Do you mean that
pawns can drop anywhere except last rank?
I do not like rule that drops cannot give checkmate because mate with
drop is such big part of shogi. If you want game to go in direction
of shogi I think pawn promote to queen is too strong and not like
shogi, maybe pawn promote to nonroyal king. Pieces going back to
unpromoted when captured is good rule. Queens gain nothing by
promoting, knights gain more than rooks or bishops, this is possible
balance problem but possibly not.
Good effort. Chessgi still has advantage it is playable with two
standard chess sets. Chogi needs way to show promoted piece from
unpromoted, but is not very big problem I think players could put
checker under promoted piece.