[Regulator]

Mmmm...like this? Dream Chaser

or this?

Energia Uragan

Fly back stages have been proposed, built and flown; it's still an overall trade that's hard to justify. Believe me, it's been done ad nauseum.

Stratolaunch is building a TSTO with flyable first stage (um, "airplanes"), and that seems to be the real way to go until somebody does a decent SSTO like Linear Airspike or the Skylon. Which may or may not work...but it seems like the ultimate best way to go with current engines.

Falcon 9 recoverable will eventually work, and for better or worse that's the path they took. Will it really be commercially viable? Bit more complicated...

[cripplecreek]

This deep gravity well we sit in is always going to be a problem for rockets.

We need a space elevator but that costs a few bucks and presents a different set of problems.