The most exciting thing about Dallas’ first Riverfront Jazz Festival is that it’s a wildcard. The event’s website, for example, includes a video unveiling the fest’s lineup with a montage of the artists’ headshots backed by the chintziest smooth jazz you’ve ever heard (Najee’s “Betcha Don’t Know”). Yet the jazz/blues/soul/R&B lineup, which includes local torchbearer Erykah Badu, is filled with enough surprises and serious jazz names to silence any would-be cynics. No one really knows what to expect. Spanning three days and three venues over Labor Day weekend, Riverfront Jazz Fest could be a flop — or it could spark a fresh appreciation of jazz and its many offshoots in a new generation of local listeners. The fest likely will fall somewhere in between, yet seeing how it all unfolds in real time promises to be a thrill.