I really like him. So, when will he show up again? Hopefully soon.
Me, too!
He came to terms with the reality of the supernatural -- or at least faced up to the reality of its existence -- in
Changes. I expect that he'll be coming back to Chicago in the wake of the Battle of Chicago: the Federal gov't
has to pay attention to a disaster of that scale (whatever the cause), so Tilly is a "natural" for Jim to bring back in this regard.
My hope is that he transfers out of the FBI into the Library of Congress' Special Collections Division.
In my headcanon, these are the Federal agency that acted like Murphy's SI (and the Black Cats before them); but on a Federal scale.
The supernatural came too far out of hiding to genuinely be swept back under the rug (though it seems some are still trying), and Tilly will (now that his eyes are opened) be able to recognize the coverup... may in fact be tasked with trying to
advance the coverup! But also assigned to figure out (post-facto, as an outsider to the conflict, with very-little material evidence, and with mortal witnesses who may wish to deny the reality and/or have been subject to various glamours & even mind-altering magics) just
what the F--K happened, and
who were the aggressors, and
who were all the other parties, and
what it was about (and
etc etc etc).
I think this could be part of Twelve Months, if Jim decides to make it so (or it might be deferred to another book).
But WoJ was (IIRC) that Battle Ground was kind of a breaking-point in the supernaturals' "masquerade," that too many mortals would have too much public evidence, and that the nigh-universal "official" dismissal of supernatural causes wouldn't be possible any longer; this was an inflection-point in the series, in the setting, that Jim had long planned.