While suing any prosecutor or police or police officer for malicious prosecution is an uphill battle, it's not impossible. While I don't know all the merits of what such a suit might look like, I can speculate on a couple of things. In reading the probable cause the sergeant who originally charged these officers wrote, it appeared he cut and pasted a lot of the information. If he didn't provide exculpatory information to prosecutors, ie; the witness testimony that Nero showed up after the initial arrest when Gray was in handcuffs and the only time he touched Gray was to get his inhaler or lift him up because he couldn't breath, then he certainly could be culpable. (I'm not claiming that happened).
If the prosecutors, after reviewing the evidence, realized there was not probable cause but proceeded anyway, they then could be culpable.
This information certainly could come out during depositions.
Having said that, I doubt any attorney would sue this prosecutor or the cop who filed the charges, but I think it'd be a very interesting process with a lot of dirt coming out. I imagine there had to be conversations about not believing this cop was guilty. This all is jmo