I think they were talking about an Unconvention. The type that Australia and NZ have, generally mostly involves turning up and socialising. Although there might be some suggestions of what to do; they are just suggestions.
The annual convention began as a way to celebrate the BookCrossing Queen's Birthday, and the one in St Louis, while having a bit more structure to it, was actually the second held that day.
Sometimes the dates change for convenience, and I don't think that there is anything set in stone as to what is required.
My biggest beef is that convention gathering places sometimes end up costing the organisers a lot of money that isn't necessarily recouped by actual attendances. The Kansas City Unconvention coincided with the worsening GFC and near bankrupted the organisers who had optimistically booked a couple of hotel ballrooms.
Not having a fixed venue is kind of liberating, and if some school or church hall can be found to donate the use of its premises for a weekend, that solves a lot of problems.
Convention hotels aren't required. BookCrossers can just pick their own level of accommodation. Sometimes I stay at a youth hostel, sometimes a fancy hotel, sometimes an AirBnB.
The recent Stewart Island Uncon was a case in point. Most of us camped in the local backpackers and had fun doing jigsaws and drinking wine, a married couple staid in the island's grand hotel, and two others shared an AirBnB and had more fun than anybody else realised. (The latter two have now changed their FB statuses to "In a relationship", so I'm not giving away any secrets.)
There were various activities planned - island tours and stuff - but not everyone did all of them, and the Sunday was mainly spent bumming around the hostel avoiding the rain, and attending the pub's trivia night where even more fun was had.
The key point of any BookCrossing convention is not the venue, nor the books, but the BookCrossers. Fun is had, friendships are made, and there are human beings now alive who exist only because two BookCrossers hit it off at a convention. (I'm thinking of tzurriz and jfroebe here, who have three wonderful children, but there are other couples, some of whom are now parents.)
Romance aside, there are few things more satisfying than wandering around a strange town in company with a herd of BookCrossers, leaving books in odd locations, and rejoicing over each catch.
So, for anyone contemplating a bid for 2020, what really matters is not an impressive ballroom or an exotic locale, but a range of accommodation, and maybe a place to put a couple of tables for the books.