May be OP is planning to support multiple platforms in future, and not just Trello?
However, I'm gambling that the benefits outweigh the impact of those risks in this situation. Building Corrello on Trello gave me access to a large user base (5M as of last autumn) and one which is growing. Check out google trends for asana, basecamp and Trello https://www.google.co.uk/trends/explore#q=trello%2C%20baseca... Not directly linked to their user numbers but Trello looks to be increasing in terms of awareness. I believe there are a large amount of people out there who were previously managing projects in excel or on post it notes. Trello has pulled those projects into the cloud and put a decent API on them. That's a whole lot of projects which were previously stuck to peoples desktops (literally in some cases) which are now accessible via an API. I think it's worth the risk that Trello decides to replicate this functionality as there is a good opportunity there if they don't (or until they do)
Good luck with Corrello - it looks great.