Seeing the screenshots,looks like you know how to work with the background,but the old graphics are bad.
Try changing the graphic colours(like turning shadow´s stage blue instead of red,or hard stage with grey sky,something like that),because vanillla graphics are repetitive and not new.If you change at least the colours of the stages and enemies(like the junk golem,with purple colour instead of grey),they will look new for the players.
Also,why not chaging which enemies appears in certain stages?(like using top enemy in gemeni for example)certain it will makes the stages even more new.But apparently is to late to do so.
if you do those changes , your hack will be even better . It´s just suggestions and not criticism