Urban Digital Twins (UDT) have become ambitions for many cities globally, yet their implementation varies significantly in terms of governance, openness and their relation with external stakeholders. Currently, many UDTs still rely on manual updating, leading to discussion on whether they really are digital twins by strict definition. To maximize their utility, UDTs must be developed to meet the multiple needs of the cities (e.g. decision making) while ensuring continuous updates and adaptability. In this work, we present a development agenda for UDTs, synthesizing insights from both recent literature and the practical experiences gained through various projects carried out by Forum Virium Helsinki and the FinEst Centre for Smart Cities. In this work, we identified four key elements for the future development of UDTs: 1) establishing common understanding, 2) modularity & interoperability 3) presentation agnosticism and 4) the social dimension.