Is it feasible to redesign the Space Launch System to use a pair of reusable Falcon 9 first stages instead of the five-segment solid boosters now being developed? The reason would be to lower launch costs while still retaining a Saturn V class capability.
But could those creatures get married rationally? For example, would having to fuel the boosters with kerosene while the main stage needs hydrogen mess up the launch pad infrastructure? And SLS is vertically integrated while Falcon is horizontally integrated. And do the attachment points of the Falcon Heavy common cores and the SLS+boosters match each other? And whatnot. Is there any obvious deal breaker?
Some key figures through Wikipedia:
Solid 16,000 kN each
Falcon 7,600 kN each
Solid 269 seconds
Falcon 282 seconds
Solid 124 seconds
Falcon 162 seconds
(Some blogger loosely through out this idea because of the recent 2016 November NASA RFI about making SLS cheaper. But I can't find that source now to be credited or blamed.)