Potential partners need an initial roadmap of how the ecosystem will develop.
Such a roadmap will provide the information to your potential partners of how they fit into, contribute to, and benefit from the emerging ecosystem. It will encourage potential partners to engage and invest. The initial roadmap cannot be too prescriptive. It will evolve over time.
But the ecosystem leader needs to provide some signposts to guide the partners on what role they can play in the ecosystem. And as always such a roadmap will be effective only when it is communicated well. You may need a portfolio of communication platforms, including social media platforms, conferences, etc.