So, because I have completely failed to find any sort of "Alternate History Spacecraft" thread where I could ask this question (that wasn't several years dormant, at least), I'm going to ask here.
I've been thinking about "post-Shuttle" spacecraft designs, inspired pretty heavily by Eyes Turned Skywards and Boldly Going - specifically, the idea of combining the "X-33 derived fly-back/fly-forward first stage" of Starclipper (from Eyes Turned Skywards) with the "optionally manned fully reusable second stage" of Shuttle II (from Boldly Going).
My primary questions are 1), how feasible would it be to swap the aerospikes on the Starclipper first stage for RS-25 or RS-68 derived engines (assuming this would also mean X-33 operates with non-aerospike engines), 2) how feasible would it be to scale Boldly Going's Shuttle II (which is either 8 crew or ~15 tons of payload to LEO) up to carrying OTL!Shuttle sized payloads (assuming you got those payloads in the unmanned configuration), or 8 crew and a lighter logistics/satellite payload?
Basic idea is the first stage would be shaped roughly like the X-33/VentureStar, but built exclusively as a fly-back first stage. Thrust would be provided by conventional bell nozzle engines, ideally ones with minimal alteration to existing engine hardware to allow for an earlier first flight. Second stage would be closer to the Shuttle II in Boldly Going - normally unmanned, cargo bay/mount sandwiched between LH2/LOX tankage, non-toxic RCS and APU, RL10s or similar small cryogenic engines for OMS (supplemented by large RCS thrusters for fuel settling/small maneuvers). Launch profile more or less as seen with ETS!Starclipper - first stage boosts second stage and then separates to glide forward to a landing site in Florida for normal launches, while the second stage air-starts its main engine and carries on to orbit.
Both stages would be using metallic TPS, and composite tanks would be used where feasible (ie., the first stage LH2 tank would probably be similar material to the Shuttle LWT or SLWT, but the others could potentially be composite if they could pass the pressure tests and such). Engines would likely be RS-25 derivatives if possible - 35:1 ratio nozzles on the first stage to optimize for its purpose, while the second stage could either retain the standard nozzle or be upgraded with a deployable nozzle extension to improve vacuum specific impulse (possibly raising expansion ratio to ~150 - 180:1?).
This idea is probably not particularly feasible to get backing for in the 1980s/1990s, but I would be pleasantly surprised if it could replace the X-33/VentureStar SSTO concept without too much finagling if you handwave how the original idea is conceived.