If the GOP collapses, I can see the Democrats picking up people in the middle, but an opportunity opening up for a party a bit to their left to bud off, focusing on the leftmost legislative seats. It might be a path to getting a decent SocDem party in the mix? This is all just theorizing, ofc.

Thats exactly the point. The democrats could choose to appeal to the left, and considering the popularity of numerous isolated left-wing policy proposals, along with the popularity of Obama’s progressive rhetoric during the 08 campaign, the party would almost certainly benefit from such a shift.