We had a family reunion in Branson three years ago. Lots to do (Musical shows abound, Silver Dollar City, beautiful lakes for boating and fishing, golf courses, etc.) wonderful time, but Biscuit is right about lots of slow moving traffic. Las Vegas has this also.
Missouri is more centrally located, and is about in the middle of the country. St. Louis and Kansas City are great spots, too. The "big thing" is having someone in the "chosen spot" familiar with the area willing to organize our Basset Reunion.
By the way, my husband thinks Alaska would be a great location for a "Waddle In".