Ferro was willing to show up to Bianca's ball. I think Harry's wedding is a bigger deal than that, tbh. It may be an either/or thing between him and Vadderung, though.
As far as people to expect to show up, I'm expecting most supernatural nations to get an invite to send representatives. It is good to shore up Accords relations if nothing else.
Something like the following seems reasonable to me:
- Summer - Sarissa and Fix, possibly Eldest Gruff
- Winter - Mab, Molly, and retainers
- White Court - all the living named characters here except maybe Thomas
- White Council - token pair of senior councilors (say Listens to Wind and Ancient Mai with TC's nameless wardens. Nobody that's gonna be too friendly to Harry)
- Archive
- Marcone/Gard and possibly the new Hendricks (Childs perhaps?)
- Svartalves - Etri and his sister
- Vadderung or Ferro, but probably not both
Probably a good portion of that is wrong, but I could see any of those choices actually showing up.
For Twelve Months, it's going to be a different kind of book. Instead of Harry's worst weekend, it's going to be stretched over the course of an entire year. That means that Harry gets time to rest between episodes of interest. I think that might make it hard to keep up a usual murder mystery plot. The usual time pressure is going to be off, anyway. Stepping back and having a less combat focused book would be good after BG, too.