About the point that the combo system gives variation to scores: it is possible to make a combo system at the same time that serves the same purpose, but without making the score so dependent on it:
For example: The current score given per hit is
Hit Value * (1 + Combo multiplier * Difficulty multiplier * Mod multiplier / 25 )
An alternate could be this (I don't say this would be a good formula, it is just a simple one to illustrate the point):
Hit Value * (1 + LOG10(1 + Combo multiplier * Difficulty multiplier * Mod multiplier / 25 ))
With old formula (scores are relative to each other): If anything that is not a combo break is a 300.
1000 combo full combo: ~20million
900combo + 99 combo: ~16million
500combo+ 499 combo: ~10million
1000 combo full combo: ~0.84million
900combo + 99 combo: ~0.82million
500combo+ 499 combo: ~0.78million
With combo effect decreased then accuracy becomes more important (while combo still has enough effect to generate variation).
About the fact people can beat other players by ignoring circles in hard to aim maps, then there can be some changes in what misses do. A very simple change would be changing misses so they reduce score by 6000 and break combo instead of just breaking combo.