The problem is that it would be deserialized as a list; this breaks the general expectation that serialization formats should round-trip.

(yes, tuple already does this; but I think it is less of a problem for tuples, since the list API is a superset of the tuple API except for hashing)

So, perhaps we could ship an optional serializer (under which form?) accepting any sequence type (and perhaps any mapping type?), but it shouldn't be the default.