Here is a cool page describing a proposed "massive" modification to the X-15. In 1969, the Marquardt Company outlined the design for a supercharged ejector ram-jet (SERJ) that used small rockets to pump the ram-jet. The web page provides more details. It also provides a view of stretched and delta-winged X-15 designs from North American.
Last summer, I saw a large model of a delta-winged X-15 at MDRA. Very cool model, but unfortunately it didn't fly as well as it looked. More photos can be seen in the MDRA photo archives (scroll to the bottom).