But it would raise other problems. Right now, the Paragon and Renegade scores are essentially the charm and intimidate scores of yore. If you replace them with a single charisma stat, then what role does the paragon/renegade score play? Nothing, basically, and they'd need to come up with something entirely new. And any alternatives would likely be either A: become stat-based gameplay things, which would suck, or B: turn into "keys" that unlock story-based events or options. Which brings us right back to your original problem.

I suppose you could try to have options that require some sort of middle ground (IE, this conversation option only unlocks if you have less than 50% renegade AND less than 50% paragon), but then the trick is to make that compelling. Which would be very difficult. Your choices become badass rebel, impervious soldier of order, and... something in the middle. The sorta badass, sorta righteous dude who's not particular strong in either category.

I think that this is an unsolvable problem, basically. There is no solution that wouldn't involve devaluing other parts of the game.