Indeed, but it means that they could be a part of a future plot without any kind of problem or lack of logic.
But we live in a world where Akuma is part of the story of Tekken 7, so who knows different companies will start merging their IP's into one universe someday.
Except again, The Ninja Gaiden characters(even including Ryu Hayabusa) still don't factor that much into the plot because ultimately just like how Ninja Gaiden is Ryu's game, Dead or Alive is Kasumi's game and it will be more of problems that Mugen Tenshin can ultimately handle with or without Ryu's assistance. Hell, DOA5 is the first DOA series where Ryu becomes way more involved with the plot past him killing Bankotsubo and how he suddenly saves everyone's asses in Dimensions.
But to be honest about the whole canon thing:
The Virtua Fighter characters are about as Canon as Akuma is: Where they have some presence in the overall universe, but nowhere near enough to really be a massive factor(I doubt Akuma is really going to factor that hard into Tekken's world)
Naotora CAN become easily canon within the Dead or Alive universe if only Koei Tecmo pretty much comes out with the fact that the events of Samurai Warriors ultimately happened within the DOA universe(which really won't be too hard, even with the presence of Fiends and actual otherworldly creatures because we had that with Warriors Orochi)
Mai Shiranui on the other hand is purely non canon. Because despite what others say she CAN be added into the typical world building(due to the fact that Samurai Showdown actually(okay maybe) has one of her ancestors in that game possibly, making it canon to the Fatal Fury/Art of Fighting/KOF universes and if people want to know, it's this guy
But otherwise since they aren't revamping the storyline or anything like that, she's a pure guest character just like any other guest character.