First, don't get yer hopes up, this isn't an actual project that is happening. This is just a personal dream I'd like to do someday.
Anways, what would you change about it? My main reason for this at all, is cuz I feel like they could've done better graphically at the time. I feel the game could easily support horizontal paralax. I don't recall it ever having any vertical scrolling, plus, there's not alot going on onscreen, so sprites could be used for paralax as well. Also, I've thought about if we ever did this, I'd probably want to use alot of tiles & sprites from Monster Lair, Dynastic Hero, & New Adventure Island to improve the look in general.
Musically I'd make it mostly redbook, but there would still be some chip tunes. I thought maybe in shops, I'd leave those as chiptunes, & then possibly add an adpcm sample for a few phrases for the shop keeper to say like "thank you". I'd also keep the tune that is used in the castle where Mouseman is falling down(which I believe it from Wonderboy in Monsterland). Id' maybe make different sounding versions of the castle themes, that could work with each castle's theme. And probably make a an alternate version of the beach tune, to use for when the screens that are accessible to Pirahnaman.
As for the gameplay itself, I'd like to change the way the character is hit. In otherwords, I hate how he bounces around when hit, that's always been a major gripe with me. I'd maybe like to improve the battle system ever so slightly. At the very least, make the swords slightly longer(especially the mouse's), though, I'm sure that would also require some adjusting in the difficulty has well to balance it out. I wouldn't mind a downthrust, but no matter what would want to keep Tigerman as the top swordsman, since, that's his main advantage.