In the Theravada tradition is the Pali Canon viewed as the final truth? In other words, are the Buddha's teachings viewed as the final authority which cannot be revised or are his teachings subject to possible enhancement or even revision by later enlightened ones in the way Newton laws were later made even more accurate by Einstein's work?
The pali canon actually includes work that is not actually from the Buddha. The abhidhamma for eg are full systematic explanations of the mind and the practices that was created by the sangha to systematically arrange the Buddhas teachings, it is in of itself not the words of the Buddha. Similar with the jataka and the dhammapada, in fact afaik the entire Khuddaka Nikāya is not specifically the words of the buddha although it is a part of the tipitika. The 4 nikiyas ARE the teachings of the Buddha, of course arranged by the sangha themself but not in a ontological sense like the abhidamma was.
As for a next Buddha adding to the past Buddhas teachings, not so much as the Buddha explained the full dhama and this dhama is ultimate truths that a next Buddha will himself find. There is no addition to it by definition of it being the highest ultimate truths about reality. There is certain requirements that a Budddha has to do to be a continuation of the previous Budddha and there are predictions about it like what tree he will sit under to reach liberation though matreya is the only one the Buddha predicted.
One would think that Canon is that and most will tell you that it is.
It is my impression that in practice one will find that some monks might more so go by what their teacher taught, might not care too much about what is canonical and may be looking for 'late additions' or 'fake texts' where canon is disagreeable to them if they were to study it.
If one says 'I don't like the Theravadin Abhidhamma, i don't like the commentary and i think some Suttas might be fake'. It wouldn't be difficult to find a group that will take him in as long as one conforms to what the founders of that lineage taught.
The Buddha said that he has taught all that is necessary for liberation. He may have known more than what he taught but those things are unnecessary for liberation.
So there's no need to search for more teachings beyond what the Buddha himself had taught in his lifetime.
On one occasion the Blessed One was dwelling at Kosambi in a siṁsapa grove. Then the Blessed One took up a few siṁsapa leaves in his hand and addressed the bhikkhus thus: “What do you think, bhikkhus, which is more numerous: these few siṁsapa leaves that I have taken up in my hand or those in the siṁsapa grove overhead?” “Venerable sir, the siṁsapa leaves that the Blessed One has taken up in his hand are few, but those in the siṁsapa grove overhead are numerous.”
“So too, bhikkhus, the things I have directly known but have not taught you are numerous, while the things I have taught you are few. And why, bhikkhus, have I not taught those many things? Because they are unbeneficial, irrelevant to the fundamentals of the holy life, and do not lead to revulsion, to dispassion, to cessation, to peace, to direct knowledge, to enlightenment, to Nibbāna. Therefore I have not taught them.
“And what, bhikkhus, have I taught? I have taught: ‘This is suffering’; I have taught: ‘This is the origin of suffering’; I have taught: ‘This is the cessation of suffering’; I have taught: ‘This is the way leading to the cessation of suffering.’ And why, bhikkhus, have I taught this? Because this is beneficial, relevant to the fundamentals of the holy life, and leads to revulsion, to dispassion, to cessation, to peace, to direct knowledge, to enlightenment, to Nibbāna. Therefore I have taught this.
“Therefore, bhikkhus, an exertion should be made to understand: ‘This is suffering.’… An exertion should be made to understand: ‘This is the way leading to the cessation of suffering.’”
The point is the bigest data, highest experience, highest trust, highest level of influence to people decision by the Buddha's practice along four uncountable kalpa plus hundred thousand kalpa of the Buddha.
No one has same practice, but the Buddha.
So, the final authority is the Buddha and people who practice hardest follow him in 8 mastery Jhana of 40+ practices, 8 specialist knowledge and entire tipitaka pali professional memorizing, according to VN1 VeranjaKanda, DN28 SampasadaniyaSutta, DN10 SubhaSutta, and VN Pātimokkhaṭṭhapanakkhandhaka.
Someone may think the authority is Dhamma, but the truth is some dhamma is wholesome, some dhamma is unwholesome. We don't really know which is really wholesome which is really unwholesome without the bigest data, highest experience, highest trust, highest level of influence to people decision by the Buddha's practice along four uncountable kalpa plus hundred thousand kalpa of the Buddha. And we can practice follow people who practice from the past generation professors to the previous generation professors who practiced inadvance of the 8 mastery Jhana of 40+ practices, 8 specialist knowledge and entire tipitaka pali professional memorizing.
Nothing easy but it's really worth more than Einstein's work uncountable times.