WOJ is that Fitz unlikely to appear again, but I would like to think that Fitz finished his education, got his GED and apprenticed to Nick Angel, helping to find kids, meaning that he is now ready to work for Dresden Investigations and the Paranet as an investigator, specialist in ghosts, getting further training and honing his gifts with Mort and Harry.
The BFS building is a big building, Harry needs staff, the Wardens are stretched too thin, so paranetters in support capacities would make sense.
You may get your wish on Cowl, we are near the time under WOJ that Cowl will be unmasked, although the split and additional books may have skewed that.
Elaine? No, I think if Harry pushes reform of the White Council and the Black Council becomes widely known, you may have hidden Wizard Level talents like Elaine and Mort step forward (Carlos once again betrayed by a woman), to avoid being associated with the Black Council.
I think it likely one or more of Harry’s Rogues gallery will turn up in BG, working for the Fomor.
As regards the whole trapdoor thing, Harry has been so homesick that of course he would spend the first night possible at home, and he deserves a nights sleep without nightmares, it has been indicated in both PT and in the Mister Microfiction that he suffers nightmares on a nightly basis.
You will almost certainly get the Merlin I would think