You know what's a pretty worthwhile beat-em-up which could have been a way forward for the genre? Jet Li: Rise to Honor, a PS2 game by Sony with an innovative dual-stick brawling system. It didn't have a jump button AFAIR, so it's got that against it, but it does tackle multi-enemy combat in a way that most 3D brawlers never figured out, and that would have been a crucial innovation for the genre to survive in the move from flat-plane 2D combat to multidirectional 3D brawling. In the game, you used the second stick to strike at enemies behind or to the side of you, and you had systems for interacting with the environment or multiple enemies at once. It used 3D to its advantage rather than just employing the graphical upgrade of 3D worlds/characters; here, the world was real, and you had to be aware of and use the full range of space to wipe out your foes.
Not a great game in total (the fighting wasn't completely balanced, and it also had some Max Payne-style shooting and other clumsy stuff,) but an interesting fighting system that never really lived up to the full potential. (The Mark of Kri series played with this directional idea too, also PS2, also by Sony.)
(One downside of this system is that it wrecked your thumbs tapping the analog stick left and right for every combat move...)
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