It isn't perfect since its a young project, but I've attempted to simplify and modularize the process of creating/maintaining forms to allow for that part to be crowd-sourced as much as possible (and I'd love your help!).
What I would really like is to somehow extract out a representation of the tax logic into something separate from the business of collecting the data from the user, or even calculating the results. I'm not entirely sure what that would look like, maybe it would have to be some domain-specific language (eww) in order to be able to fully express the relationships and dependencies between different fields/forms.
But I think that tax logic portion is the difficult-to-maintain part, and also the portion different projects would benefit the most from sharing.