”’Mamma Mia’! Now and Forever!” It just ain?t got the same ring. The helicopters and singing kitties of the ?90s have faded into memory and the Disney marquee is crumbling (”Aida” gets buried alive Sept. 5), but a new juggernaut has yet to arrive. Last season saw disappointing revivals (”Fiddler on the Roof,” ”Little Shop of Horrors”) and British imports (”Bombay Dreams”); ”The Boy From Oz” closes when Hugh Jackman leaves Sept. 12; and ”Hairspray” lost 10 percent of its audience along with Harvey Fierstein. If the biggest hit of 2004 is literally the smallest — the puppet-driven Best Musical ”Avenue Q” — what?s happening to the Big Broadway Blockbuster? Producer Barry Weissler, whose eight-year-old minimalist ”Chicago” is going strong (comic Wayne Brady starts Sept. 7), says people simply prefer performance over production: ”The audience shifted gears and traveled down a simpler road.” Maybe all that jazz is all you need.