I posted about this yesterday in the Big Berserk Exhibition thread, but I think it warrants its own dedicated discussion.
Some information about the area of the exhibition where they set up a reproduction of Miura's workspace. Apparently you can see 12 color illustrations from Miura on the desk, produced as part of a giant robot animation project. The animation planning studio was called "Metabaron" (no doubt a nod to the Jodorowsky/Gimenez graphic novel series) and was set up around Miura, who loved the giant robot genre. Chica Umino, Kouji Mori and Takashi Hoshi were also involved. Umino was doing the character design.
The project is said to be "rife with Miura's ideas", from the character outlines and the world view to the scenario. A visitor said on Twitter that the robot's form is reminiscent of Mitsuteru Yokoyama's work (Tetsujin 28-go, Giant Robo), but more elaborate. Finally, it seems two protagonists are involved: a girl who handles the "motion" and a boy who operates it (presumably the weapons?).
More details are likely to emerge over time, but that's all I have for now. Do note that the concept art shown in the "Messages to Kentarou Miura" booklet is not related to this. It was created in 2006 for another project that was later abandoned. Again, we'll have more details over time as we get to translating those messages, but that will take a while.