Is the Libby case the end of Fitzgerald's investigation?

Looseheadprop at Firedoglake speculates that the Libby case is not the end of Fitzgerald's investigation. The possibility is raised by the seemingly strange way that Fitzgerald offered Ari Fleischer immunity without knowing what he had to offer:

Maybe I have lost my mind or am just high from WAY too much popcorn, but I think that this means that Team Fitz gambled that Ari had something huge to tell. The fact that whatever it is that PatFitz is saying he does not have to turn over to the defense even exists, to the extent it may exist, suggests to me that the investigation is not over and that maybe, just maybe, that gamble has paid off.

Please understand, this is ONLY tea leaf reading, but no other explanation suggests itself that accounts for all factors. I think maybe there is something out there, that came from Ari, that is NOT part of the proof relating to the crimes Libby is charged with (does this also explain why Libby's charges were so narrowly drawn?) that Team Libby is dying to know about and Pat is fighting hard to keep a secret. If it was all going to be over after the Libby trial, why fight so hard to keep this info secret?