Building an ecosystem/marketplace requires a broad range of considerations. Some that came to mind today when discussing the subject with someone...

1. Figure out your Personas/Stakeholders. Each one may (likely) require a different kind of marketplace, tools, messages etc.

2. If you're at an existing company, remember that you will essentially need to set up a company within the company. This is because all the functions at the company are likely focused on your traditional customers rather than your ecosystem stakeholders.

So you will need people/sub-teams in...

Product
Marketing
Ops
Support
Community
etc

3. Remember that ecosystems/marketplaces take more time to develop than you'd expect - so set expectations correctly with your leadership team.

This is because, unlike normal products, you can't just work with internal stakeholders to ship features for users. You need to ship features, then you need to work with partners to consume/act in a certain way, then you can give it to end-users. Then once you learn from user behavior, it takes yet more time to coordinate all 3rd parties in order to make changes and retest.

4. Ecosystems and Marketplaces need a wide range of key decisions and frameworks - including...

Clear definitions of stakeholders and per stakeholder:
Asset/use-case types
Distribution types
Policies/incentives/guardrails
Go to market/messaging
Community engagement programs (1:many),
Strategic partner engagement programs (1:few)