The most advanced variant of the X-15 that stood a fair chance of getting built was the X-15A-3. Where the X-15A-2 was a stretched version of the X-15, with increased internal propellant tankage and additional external tankage, the A-3 would have been stretched even further. Additionally, the wings and horizontal stabilizers would have been removed and replaced with highly swept delta wings, and the rocket engine would have been upgraded (or simply replaced with a new and/or more powerful engine). The nosecone would have been stretched.
The end result would not have been much of an increase in speed, but duration and range would have been greatly increased. As long as the A-3 was, it was still to be carried to altitude by a B-52. Like the A-2, external tankage could be carried for increased performance, but for some reason the design featured only a single centerline tank.
The photo below shows a display model of an early design of the A-3. The final design had diamond-shaped wingtip fins.