Former Creative Artists Agency President Ron Meyer, 50, today will spend his first day as president of MCA Inc. after having been a high-powered talent agent for three decades. The buzz in Hollywood continues to swirl around Meyer's former home as well as his new abode. Industry watchers want to know what will happen at CAA in the wake of his departure. How will Meyer's former partner, CAA Chairman Michael Ovitz, redistribute Meyer's share in the agency among a group of aggressive younger agents and the elder statesmen? How will the agency's management be reorganized? As for MCA, the talk revolves around who its new owner, Seagram Co. Chairman Edgar Bronfman Jr., might bring in as his No. 2 corporate officer. Warner Bros. co-Chairman Terry Semel, whose contract has an out at year-end, still seems to be the name on everybody's lips.