I think Cinder's is the design I'm most looking forward to seeing in Vacuo. We've already seen her grimm arm spread above her elbow to over her shoulder from volumes 5-9. Since the end of volume 9 implied there'd be a bit of a time skip, the grimm could have potentially spread much father. If grimm replaced her arm it could also replace her eye!
There's also this concept art from volume 4/5 where she has these cute (tiny!) arm spikes on her grimm shoulder so I hope we at least get something like that eventually.
