On the contrary, rather (as a regular adversative conjunction, introducing a word or clause in contrast or contradiction with the preceding negative clause or sentence).
However, although, nevertheless, on the other hand (introducing a clause contrary to prior belief or in contrast with the preceding clause or sentence).
Except that (introducing a subordinate clause which qualifies a negative statement); also, with omission of the subject of the subordinate clause, acting as a negative relative, "except one that", "except such that".