Normally we pay more attention to who we think would provide good panels, rather than numbers of attendees who'd show up just to see them, but after this most recent year when we ended up having to pay for function space at the hotel due to not making room block*, we really, really need to find a guest who will bring in the crowds. We had good luck in the past with Lois McMaster Bujold and Jim Butcher--someone on that level would be awesome.
They also need to live in the US or Canada, as our budget does not extend to international flights from anywhere else.
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* In part due to hotel shenanigans which were, while totally legal by the contract, not to our benefit. You can comment here or at the Dreamwidth crosspost. comments at Dreamwidth.