Call it a come-on attraction: Last month's teaser for MGM's Showgirls (due Sept. 22) promised ''a movie event so erotic ... so dangerous ... so controversial ... that we can't show you a thing.'' Then it went ahead and showed us a barely clad dancer (Elizabeth Berkley) slowly rising out of a crouch while running her tongue along a shiny metal pole. If you thought that was spicy, check out the follow-up trailer in theaters now, which is chockful of bare-breasted Vegas beauties who dance on stage, fight backstage, and get very friendly in between. The new version is being shown only with R-rated movies, but since the Joe Esz terhas-scripted, Paul Verhoeven-directed film is a well-publicized NC-17, there's obviously even more hot stuff the studio can't show us. Not that it would even if it could. After all, leaving something to our imagination is one of the oldest principles of coming-attractions showmanship.