Which reminds me, during the scene with Thrawn's reveal in the trailer, the one where he is looking at the art, (I can't find the picture but) someone made a deep analysis of the pictures and what they show. Sure enough they do deal with our main cast, but when more focus is placed in that one Mandalorian art in the back, you can see that it is an image of Mandalore the Ultimate.
For those who don't know, Mandalore the Ultimate was the leader of the Mandalorians during the Mandalore Wars, and was ultimately slain by Darth Revan, right before he allowed his general to use the Mass Shadow Generator.
And during an episode of Rebels, the crew meet Mandalorians, who apparently went back to their honourable ways and once again followed the mandalorian code. The current Mandalore of that sect offered the Rebels some tea, saying it came from the Cassus tree.
Now this doesn't in itself canonize much, other than a name of a tree, but you can see that they have some reason to call the tree such, considering it is considered to make strong enriching tea. (Which if you do believe Cassus himself is canonized, considering the Fett clan exists, then you could argue that Taris and the Rakghoul disease exists as well, considering how his story ties in with that).
About them canonizing Malachor, but having a different look and etc., I believe that Malachor V still exists, with all this new evidence and such. The planet that Ezra and the crew landed on was Malachor I. The Malachor system is home to a couple planets; ofc the only known onces (iirc) are Malachor (Rebels), Malachor III (SWTOR), and Malachor V (KotOR 2). So I think it does exist, but they were on a different planet in the Malachor system.
People really underestimate Rebels for what it really is, and when analyzed properly is really, really interesting. Little hints and easter eggs lead to a TON of expanded universe stuff to be canonized, and this is another brilliant method of Filoni; he drops these hints and stuff to not officially bring stuff into canon, but make us make the connections and believe it is canon, and he kinda keeps it in a cage where Disney can't do much about it due to his story writing.
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TL;DR?
Mandalore the Ultimate is canon-ish (just needs to be properly addressed since he is only seen in art), Cassus Fett is canon-ish (through assumption due to the Fett clan existing and due to the Cassus Tree being called such considering its strong tea), and Malachor could be Malachor I and not Malachor V.
There once was a member of this land,
A humerous posting he had planned.
But when he posted,
By mods he was toasted,
And he had found himself banned.
