Years ago, when I first heard about the concept of a dualing wielding fighter (and when I got over "in real world, the only person who could do so was the legendary myamoto musashi"), I imagined a swift, dancealike fighting style.
TWF needs a full round action, no move action possible with it. Which is the reason why all the guides say "don't do TWF - if your enemy walks away, your attack is far too weak and if he doesn't, you're still better off with a two handed weapon - with the exception of the rogue for his sneak attack".
So my suggestion is to find a way that allows movement AND the off hand attack.
I have a class like "Blade Weaver" in mind, imagine a fighter with two scimitars who moves like a whirlwind through his foes.
The idea is a class feat that allows to use the benefits of TWF (and Imp TWF, Perfect TWF et cetera) combined with a regular move
My basic question here is: Can one balance this against the other classes?
Of course there can be restrictions like "only while wearing light or no armor" and allowing Imp TWF in this feat only on higher levels of Blade Weaver so a one level dip doesn't work.