It'd be helpful if you described the game a bit so we don't need to actually check it out to come with suggestions :P (That's a pretty transparent way to try to get easy traffic, I'm not gonna fall for that).
Some generic stuff that always makes a game better:
- Screenshake and explosions. A little visual juice makes anything feel more polished.
- More playable characters that have unique mechanics. Forget "he jumps higher but runs more slowly", you want stuff like "can't attack, but can deflect enemy bullets back at them" or "flips gravity upside-down instead of jumping". Characters like this that makes a game play entirely differently can essentially make all the old stuff feel like new again.
- More levels. If they have unique gimmicks like glowy fireballs or slippery ice you can make levels feel more distinct from each other, even if you don't even get that much gameplay differences from them.
- Funny writing. You don't need to force cutscenes down players' throats, but ads for "PIZZA BURGER: THE BURGER WITH A PIZZA INSIDE" off in the background and funny flavor text for all the upgrades you can unlock can help setting the mood for a game.