I'm not as familiar with the .x file format, however I do believe there could be some problems with a "universal" system.
for instance, if a rotational value is off in the base pose, the arm might bend the wrong way. Easy fix, but still can be annoying.
Or, If a dancer requires more bones than the animation set has, for instance, if you want a ponytail or an accessory to move with the dancer, you'd have to have it use the animation set in the dancer's model file for those bones. Then you get into the issue of telling Osu! which bones to do that for, and then you have to animate those bones with the dance animations. But wait, then you'd have another problem. to animate those bones decently, you'd need to have the animations on hand so that you can do it.
If you try to solve that problem with things like a "dynamic movement" or whatever system, you get the problem of needing to set limits so that they can't go the wrong way, and/or do some collision checking to make sure that those items don't go where they're not supposed to.
If you don't allow for more than the standard bones, then you'll get characters with long hair/other things that doesn't animate nicely, and then it just looks cheap.
Besides that, I'd say that if all the dancers did the exact same dance, then it would be kinda boring because you wouldn't get to see a different take on the style/dance. Yeah, it means a lot more work in the long run for the people who have to make the dancers, but I'm pretty sure peppy has a lot better things to do than try to find a way to make a universal system that allows for as much flexibility as people would want.
I would just be happy with a set of 3dsmax files that had a biped doing the dances and stuff, and then I could just work off of those to get an idea of the style and timing, and also variate it according to the character.
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