I like where your head is at, but this feel short
This is a terrific idea and would have made for a great game but sadly this was just too glitchy and there isn't enough.
The gameplay is far to simple. You have a button to jump, one attack, and moving buttons. If I'm wrong and there are more, than I missed them. You need to explain controls better if thats the case. I played as Megaman and I beat every bad guy by simply blasting him a few times till they went off the screen. Mario just walked right off the ledge before I'd even attacked. Bugs galore.
Your main focus on making this game was the characters. The variety of characters in this game is niffty but that seems to be all you were working towards and you left the rest to flop.
So my advice to you is try again. This is a great concept, I'd love to see it in a much more playable way. I suggest gathering a team of the best programers you can. For your first game, it's an admirable try, but for Super Smash Flash 2, you're gonna need many more hands on deck to make it great so look for them. I so want to see this be an awesome game!