Using theta lifting, Gan-Ichino proved a multiplicity formula for the tempered automorphic spectrum of the adelic metaplectic group Mp(2n, A), which generalizes Waldspurger's celebrated work when n=1, at least in the tempered case. Based on the recently obtained stable trace formula for Mp(2n, A), I will sketch how can this be possibly extended to the whole automorphic spectrum, leading up to a multiplicity formula of Arthur type. This is a project in early stage, and I will only explain the basic ideas.