For a fully-reusable launch vehicle to be developed, we need a lightweight thermal protection system and method for (controllable?) reentry. This is much more difficult than reentry for lower stages such as SpaceX's boosters, which are moving at a much lower velocity.
Upper stages typically are much more squat cylinders (with much smaller rocket engines) than lower stages, but they are still very much not compact capsules like typical reentry vehicles. They also are much less dense because they consist of empty or mostly-empty tanks, and they can't tolerate much increase in mass.
Still, I understand that there are a variety of potential ways of making them reentry-capable.
Have reputable worked examples or studies that include mass breakdowns been published for any or all of the following re-entry methods?
Space-shuttle-like winged upper stage (Does not really count as an upper stage unless it provides at least 1000 m/s of dV with a payload, so not the actual Space Shuttle).
Other lifting-body or winged designs
Bicone forward-flying reentry vehicle
"sideways" reentry cylinder (this seems to be popular for NASA Mars landers, but I don't know if it's viable for Earth)
The "Kerbal option" in which the stage flies backwards, with a (probably-ablative) shield closing over the (much heavier) engine-containing end -- I name this because the available parts and physics simulation in Kerbal Space Program make it frequently used by players.
Von Braun's metal leaf drag devices
Ballutes and similar inflatables