Hate to be the pessimist here, but if it turned out anything like Buffy then I would have to stop watching Who. Buffy was torture for me to watch with my ex.
Fire Fly was much better though, still haven't seen all of it come to think of it, should get around to that.
I would say that cross over-wise I would not like to see any type of cross over. That is TW, SJA and DW I would be happy with but any other unrelated show I would not.
I would not mind seeing certain actors, but would prefer the show not turn into, say, "Buffy in Space" if they used to many.