One of my favorites is the E-75 Standardpanzer. Germany’s Entwicklung series was intended to have been a replacement for its existing tank designs. See http://en.wikipedia.org/wiki/Entwicklung_series . The Entwicklung series tanks were intended to be simpler, easier-to-produce, more mechanically reliable versions of the tanks they replaced. The series ranged from the E-5 (5 - 10 ton light tanks and armored reconnaissance vehicles) to the E-100 (a 100 ton successor to the Maus).
The E-75 was the intended successor to the Tiger 2 (a.k.a. the King Tiger). The E-75 was significantly better armed and better armored than the Soviets’ IS-2. However, the E-series tanks were still in the prototype phase when the war ended.