Well Nasa did come close to the right solution.
Here.
All that was needed to make it an American Energia would've be to remove the engines from the orbiter and cargo pod altogether, and place them under the core stage as shown on the bottom HLLV.
Direct has the potential to set the shuttle program straight, as it should've been built from the beginning. A heavy lift core stage that can launch a myriad of payloads (either stacked or piggyback) depending on how it's configured.