My 2 cent guess is cannon will be no Tav but rather Shadowheart, self redeemed, as the heroine of this story.
The only reason I doubt that is that if they were to declare a canon they'd already erase most player choices...so the custom character will likely be it, mentioned in some generic term like Saviour of Baldur's Gate or some such so when listening to it people could think "oh yeah, that was my character".
But with that said, I don't think a canon will ever be declared at all for BG3 because this is it, this is the end of the story of the dead three and this story, there will no more sequels. If there are to be other Baldur's Gate games they will be entirely unrelated stories with all new characters, etc. So BG3 will most likely be left open ended in terms of canon, with no defined main hero and plot results...whatever happened will be left to the imagination of the players and whatever paths they best liked.
