It was initially thought that Miranda had been completely shattered and reassembled several times in its history, each time burying some parts of the original surface being and exposing some of the interior. Now, however, a more mundane explanation involving the upwelling of partially melted ices seems to be in favor. This was accomplished by rotating the entire spacecraft while the camera's shutter was open to compensate for its motion. The resulting images have the best resolution of the entire mission. The upwelling and shattering explanations for Miranda's bizzare appearance are only theories.
Much more evidence is needed to provide a satisfactory explanation.
There are no more missions planned to Uranus and Neptune. The data from Voyager 2 may be all we have for a very long time.