I might fork and do my own version where I try to adhere to Swifts strengths while still staying close to Rails API where I can, e.g same naming scheme and pipeline design as Rails.
Rails biggest weakness is Ruby. Leveraging Swifts strengths with protocol extension, where clauses and generics could actually improve on Rails current API.
Edit[0]:
Added an idea for filter API. Not happy with it, but it's a start. Before/After filters could just be a list of "stuff" (selectors, closures, etc) to be called before any action. It could also use group_dispatch to ensure that filters are called in sequence and only call the method once all filters are done.
group_dispatch (think semaphores, but not as "dangerous") :)