Here's the problem with that

The Parliamentary Labour party is still very Blairite and such a move would cause a major split similar to what happened in the early 80's when a bunch of Labour centrists broke off to join the SDP, splitting the left wing vote and ensuring that Thatcher won in a landslide in 1983.

Labour is already in the midst of an open civil war as it is and to be honest, Corbyn is going to have to do a LOT of compromising to stand any chance of staying in the job.