Meteor fan here (new convert). Why build something that already exists, just by curiosity?
1. There are few web based mind mapping tools that are really good.
2. Learning new technology by making things (Meteor in this case)
3. Putting what you make in public for feedback.
4. Improving it iteratively to match the best in the market. Desktop or web-based.
5. Test the boundaries of the platform (Web in this case)
6. Feel good about it as a dev. :)
Start again with a new product. Keep learning and feeling good eternally. :)
1.) To learn how to build a non-toy application in Meteor
2.) Freemind is a desktop Java application, so having this be web based means it's available to more platforms.