I was once at a table next to Threadgill at an East Village Sicilian restaurant. Though we certainly know people in common we had never been introduced, so I didn't say anything to him. But he's probably one of the people I'd invite, probably Butch Morris too. I'd really look for people I can count on for being interesting conversationalists, not just musicians I like. Among the dead I'd probably think of a guy like Rex Stewart whose writing suggests he'd be a good talker. And Steve Lacy without a doubt. I probably wouldn't invite Monk or Bud... Maybe Ernest Dawkins because the couple of times I've chatted briefly with him he seemed like such a nice guy. Oh, and for another nice guy who's has stuff to say, Dave Burrell.
You should consider inviting Juan Tizol. And if you really want to up the stakes, Tizol & Strayhorn.
I used to work as a programmer for insurance companies, so at one point I imagined a dinner with three legendary insurance men: Charles Ives, Wallace Stevens & Franz Kafka.