Macaw will have a field day with this topic.
First, I'll copy over the description from the Frozen Utopia website:
A side-scrolling beat-em-up in the vein of Final Fight. Choose from several different characters, each with their own unique skills and special talents.
First, is the setting in the modern day like Final Fight? The word "dragon" often suggests a medieval fantasy setting.
I don't think you'll be able to satisfy everyone. If you allow juggles and want balance, then boss HP needs to be lengthened to account for that. Otherwise, experts will breeze through the game. But if you do all that, then beginners will have a hard time. Arcade beat-em-ups have focused on expert players (e.g. if you don't know solid juggles, there's 0 chance to 1CC a Knights of Valour game). That puts me off personally (hence why I don't play any other fighting game except SF2 seriously), but perhaps I'm in the minority. If you must, consider a custom combo option that uses the super gauge. That way, the system is more segmented: new folks can super and experts can custom combo to do even more damage.
My suggestion is to not over-complicate the game. There's a reason there are 0 English language beat-em-up forums. Diehard fans are few, much less those who actually understand strategy to 1CC these games. Although I would like nothing more than another game to take advantage of my 6-button PCE controllers, risking alienating all the 2-button players with obtuse button combinations won't work. I like simplicity myself, although having special move inputs and perhaps even a super bar wouldn't be bad ideas.
I actually think you have one of the toughest jobs. A lot about a beat-em-up is the visuals. Ideally, you'll be able to make the sprites as large as in Anesan. One of my least favorite art styles is Violent Storm, which made every character look really ugly. It's also dangerous to get too cartoony; I don't think anyone really likes beat-em-ups for kids. Meanwhile, games like Bare Knuckle II and III have great sprites. Remember also that enemy variety is critical. I imagine that not enough enemies is the #1 reason people find beat-em-ups repetitive.
Mechanics-wise, having at least one jump attack that doesn't knock down is a great way to make those moves useful. Innovative pickups is a weak point in the genre; something that adds a move or temporarily transforms you would be interesting. I've always liked the concept of plot choices in a beat-em-up leading to vastly different endings. BKIII and Guardian Heroes (not exactly a belt scroll) touched upon this.