As far as festivals in the US, I looooved Mardi Gras in New Orleans and would go back every year if I could.
Burning man was great for the art and expression, but the physical environment is too difficult for me. If oodles of money were available, I would consider spending a year or two designing a shelter to set up there so I could attend in comfort.
I never made it out to the Oregon Country Fair, which sounded fun.
But what I'd really love most is to just keep traveling. I'd put an ad up on this site to find someone to come care for my animals and keep the rats out of my home while I take off. Preferably in a new Mercedes Sprinter. I'd pick up right where I left off 20 years ago... stopping at every museum, art gallery, famous dead person's house tour, botanical garden, and state park.
I once met a couple who traveled the world going to all the major festivals. Their stories were fabulous. But for me, traveling was a way to connect with the community of everyday America. Community festivals like Prune Fest in Northern CA, the Balloon Festival in Albuquerque, or the Independance Day Parade in a NY town with a population of 900 were the events that defined America for me. Likewise, the American Computer Museum in Bozemzan, Frank Lloyd Wright's Cedar Rock, and the Petersen Rock Gardens and Peacock Farm in Central Oregon. There's just so much to see packed into this country - and more often than not, advertised with nothing more than a painted wooden sign in a field outside of town.
So it's hard to say what I'd love to see most... I probably don't know yet that it exists.