Excellent gameplay with a thoughtful premise
However, I do have things to critique, and don't worry. I still give you a full rating.
First off, you could have tweaked Simon's jumping better to where you can have more control to change his directions in midair. I know his jumping abilities are like that in Castlevania, but maybe you could break a few rules of the game play.
Second, there are some parts of different stages that could've been made easier for other characters to choose from. For example, at 8-1, there's a large pit, split in half by a thin ground piece. You can at least add a platform for characters like Mega Man to get a across that. Another example is at 8-4 where you're underwater, and there's a tiny section you have to go through, but there's a fire stick that makes it difficult. I know it's a part of the SMB game, but I think that should be removed because it was ridiculously difficult for me to through with Bill from Contra. I can deal with a challenge, but I personally think these should be fixed.
Thirdly, you should include an option where you game exit the game and go back to the main menu to restart your file where you left off in case things didn't go well before. I have to restart the game entirely every time I choose to do that.
Anyway, good game. Graphics and game play are all at top notch. And with the inclusion of Bill from Contra, I'm more than content to shoot the crap out of Lakitu. I always hated that bastard, floating in the air and dropping those spike koopas.