There have been many pieces in the media noting that the Republican platform calls for restoring Glass-Steagall and arguing that this is stealing an item from Elizabeth Warren's agenda. While the Republican proposal would presumably restore the separation of investment banks from commercial banks that take government guaranteed deposits, the 21st Century Glass-Steagall Act being pushed by Senator Warren goes well beyond this.
Most importantly, the act would change the priority given to derivatives in bankruptcy. As current law is written, various derivative instruments have priority in bankruptcy proceedings over other liabilities. This allowed non-bank institutions like Lehman to continue to carry through normal business even as their financial situation was deteriorating due to bad mortgage debt. By taking away the priority for derivative contracts, market actors would have serious incentive to evaluate the financial situation of an institution like Lehman. This would likely prevent it from developing the same sort of massive uncovered liabilities that Lehman did in the housing bubble years.