I don't know if improper lists are really a problem, just hackish. :) My "solution" would be to remove the need for them by changing the rest parameter syntax (both in parameter lists and in destructuring patterns).
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"how about this: proper lists would have a type of 'list and improper lists would have a type of 'cons."
I don't think I like the idea of something's type changing when it's modified. But then that's mostly because I don't think the 'type function is useful on a high level; it generally seems more flexible to do (if afoo.x ...) rather than (case type.x foo ..), 'cause that way something can have more than one "type." Because of this approach, I end up using the 'type type just to identify the kind of concrete implementation a value has, and the way I think about it, the concrete implementation is whatever invariants are preserved under mutation.
That's just my take on it. Your experience with 'type may vary. ^_^
