I think this is more of a thread to QQ about backpeddling than a legit suggestion. You really want a random chance to fall over while playing? Doesn't sound like a fun game mechanic to me.
Running forward keeps your momentum up and is better for running away and during fights. Plus running backward already has a decent downside - you run slower than even the slowest guys who run forward. As long as you stay out of kick range backpeddling is easily countered with W and is mostly a tool to see the battlefield while repositioning yourself.
Nerf W?