Many companies and even certain governments strive for centrally led systems. At first hand, this might look beneficial, but as it can easily result in value extraction, this is not in the interest of society. Rather than fining and giving financial penalties to the well-known centralized platforms for taking monopolistic positions, a better approach is to encourage and support developing viable and equitable alternatives, organized as decentralized digital ecosystems in which decision power is distributed. Blockchain technology can play an enabling role here, but there is work to do. First, blockchain systems should provide rich support for decentralized on-chain governance. Second, the expression of governance structures should be closer to the end users, rather than requiring in-depth knowledge about distributed systems programming. Third, high-level graphical modeling languages can help here, provided that automated translation of models in these languages to smart contracts is supported. We position and introduce the DECENT software modeling toolkit for designing decentralized governance. This allows for designing, analyzing, and exploring decentralized governance for digital ecosystems. We explain the design notations of DECENT and use the toolkit to experiment with the case of the Digital Euro.