Marion Merrell Dow's stock, long juiced by takeover talk, is up anew on rumors that Hoechst is circling. What could Hoechst want? With patents expired on its Cardizem CD and Seldane-D drugs and with sales of its Nicoderm patch weak, Marion's profits should be $319 million in 1996, down 54% from 1992, says A.G. Edwards analyst Ken Nover. But Hoechst may see an inroad into managed care. Marion has earned a loyal following of HMO pharmacists, and its Rugby generic drug unit is strong. Nover says Marion may fetch $28 a share. Hoechst wouldn't comment.