As of metaphysics, Kant explicitly wrote that there can be other contributions and even expected them. More problematic is the question whether he expected his own transcendental philosophy, to which the transcendental deductions belong and which is a meta-metaphysics, to be revised or amended. If we take it to be a method rather than a definite content, as later Kantians did, this certainly is possible. But I would question whether Kant himself would have been open to such a view.
1. (Legitimate) metaphysics by other authors than Kant himself?
Let me quote from the introduction to the Cambridge Edition (2004), more specifically the synopsis. First from p. xxviii (all bolds are mine):
General Question (§5). Kant restates the question as: “How are synthetic
propositions a priori possible?” The existence of metaphysics as science depends on a successful answer to this difﬁcult question, which belongs to “transcendental philosophy,” a science that precedes metaphysics and
determines its possibility. The “main transcendental question” is further
divided into four questions: the ﬁrst two respectively ask about the possibility of pure mathematics and pure natural science, the third asks about
the possibility of metaphysics in general, and the fourth asks about the
possibility of metaphysics as science.
So basically, the main thing he did in his Critique was transcendental philosophy, not metaphysics, i.e. the main work of filling the metaphysical gaps is not done as of yet. This is further supported later in the book (p. xxxii):
Solution to the General Question: “How is metaphysics possible as science?” (pp. 116–22). Kant asserts that it is possible only through a critique
of pure reason, which must set out and analyze the entire stock of a priori
concepts; which must refer such concepts to the various sources for their
cognition (sensibility, understanding, reason); which must “deduce” the
possibility of synthetic a priori cognition; and which must determine the
principles of and the boundaries for the use of all a priori concepts. Kant
hopes that the Prolegomena will excite investigation in this ﬁeld, because
metaphysics will not go away, given reason’s natural impulse toward metaphysical speculation.
Appendix (pp. 123–34). Kant proposes that the best route to rendering
metaphysics as science actual would be a full examination of the Critique of
Pure Reason. He defends the Critique against the Garve–Feder review and
its charge of Berkeleyan idealism, and he proposes that the Critique and
these Prolegomena be made the basis for working out a new metaphysics,
limited to the principles for possible experience.
This synopsis makes clear that while he thinks to have laid the foundation for metaphysics as a science via his transcendental philosophy, there is much left to be done with regards to "metaphysics as a science". Namely, it has to be fleshed out, filled with metaphysical entities that fit within the boundaries of this new kind of critical metaphysics.
His seldomly acknowledged work Metaphysical Foundations of Natural Science as well as, obviously, the Metaphysics of Morals are two examples of that.
2. Other transcendental deductions?
This does not mean that the transcendental deductions are up to revision or alternative approaches, though: This is transcendental philosophy par excellence, i.e. meta-metaphysics. And I doubt Kant would have said that this is up to discussion.
On the other hand, the deductions are mere justifications to lift a given proposition beyond any reasonable doubt, so there is no logical reason which forbids other transcendental deductions for the same synthetic propositions a priori.
Spinning this thought further, one may think that if we see transcendental philosophy merely as a method rather than a canon of unquestionable truths, a change in the body of experience (e.g. per science) may make significant revisions necessary. This is what philosophers after Wilhelm Dilthey (historicised a priori) thought to be modern Kantianism.
As for Kant himself, I doubt this to be a possibility for him. Firstly, he thought of sciences proper as giving us necessary truths. Secondly, synthetic propositions a priori are, as the very name says, logically prior to all possible experience. He could not conceive e.g. Newton's Laws ever to be relativised and turning out to be only half of the truth. There was a great optimism in his time that science finally got hold of the deepest and eternal truths about nature.
Thus, I'd argue that while it is Kantian to allow transcendental philosophy to be seen as mere method and revise the project, including new deductions, Kant himself arguably wouldn't have been open to such "atrocities".