So this could go a number of ways...
One might argue that the term "EVO" would remain consistent with the build of a 4.3" screen and a similar form factor. The device just continues to "EVOlve" into a newer, better device.
Another possibility is that the term "EVO" could be used like Motorola and Verizon's "Droid" marketing campaign. Therefore, an EVO could be any number devices, be it the original form factor, or the rumored "Knight" device with a 3.7" screen and a slider.
I personally would like to see an HTC phone with a 4"-4.3" screen (AMOLED would be nice) and a slide out keyboard, and that is thinner than the Epic. That would be the mixture of hardware that I would prefer in a phone.