There's one particular name which shouldn't come as much surprise, considering my screen name, but Paul McGillion would be an excellent guest. I kinda feel a bit greedy even suggesting potential additions considering how large the guest list is at this point, especially for the cost of the ticket. I already mentioned him in another thread, but Patrick Currie is another who hasn't been to many cons in relation to the headliners.
There is, of course, for SGU representation Louis Ferreira who stars in a show being done (AND set, in a rare occurrence) in Vancouver. I would also suggest Robert Carlyle, but from what I understand he does not do conventions with the exception of San Diego Comic Con.