Here's another reason you couldn't have previous protags show up: gender options. There was no choice in RBY, you had to be Red. So there was no-one else who could appear in Mt Silver. But I recall when HGSS came out people were frantically speculating as to whether Leaf would appear in Red's place. It was honestly quite relentless for a time.
Early reports are saying she doesn't appear.
Leaf truthers: "well, maybe if you play as a girl, she'll be there!"
Nope, doesn't happen.
Leaf truthers: "But maybe if you beat Red 10 times Leaf will appear!"
She doesn't.
Leaf truthers: "Perhaps it's a 1 in 1000 chance!"
It's not.
Leaf truthers: "She might be in the game code!"
She isn't.
Leaf truthers: "There might be DLC!"
On and on and on it went.
But my point is this: all games after Gen I have have had a choice of protagonist. So if one of them ever showed up, you'd have to declare which of them was the canon protagonist and which was the NPC rival.
You can see why Game Freak might be reluctant to do this. It brings a whole new trickiness: as well as having to choose a canon six Pokemon, you have to declare whether the boy or girl option is the "true" one. And that's a can of worms Game Freak doesn't want to open. So all of the games which have referenced post-Gen I protagonists have always been very careful not to suggest who among them was canon. Cynthia describes the protagonist of the Sinnoh games as "that trainer", leaving it nice and ambiguously open as to who it was. Steven vaguely mentions "all the trainers who give [him] tough battles", not specifying who. If no Memory Link is used in B2W2, N just talks aimlessly about "that trainer" as well.
Sometimes less is more, and I firmly believe that this is a case in point. As interesting as it is that the BW protagonist remains absent in B2W2, allegedly voyaging in a far-off region searching for N (and possibly getting into brand-new adventures) I don't expect or hope that we'll ever see it. It is far, far more interesting to have that be left unseen than it would be to show it - and it wouldn't really add anything to the overall Unova plot unless we ever get a third installment.
Early reports are saying she doesn't appear.
Leaf truthers: "well, maybe if you play as a girl, she'll be there!"
Nope, doesn't happen.
Leaf truthers: "But maybe if you beat Red 10 times Leaf will appear!"
She doesn't.
Leaf truthers: "Perhaps it's a 1 in 1000 chance!"
It's not.
Leaf truthers: "She might be in the game code!"
She isn't.
Leaf truthers: "There might be DLC!"
On and on and on it went.
But my point is this: all games after Gen I have have had a choice of protagonist. So if one of them ever showed up, you'd have to declare which of them was the canon protagonist and which was the NPC rival.
You can see why Game Freak might be reluctant to do this. It brings a whole new trickiness: as well as having to choose a canon six Pokemon, you have to declare whether the boy or girl option is the "true" one. And that's a can of worms Game Freak doesn't want to open. So all of the games which have referenced post-Gen I protagonists have always been very careful not to suggest who among them was canon. Cynthia describes the protagonist of the Sinnoh games as "that trainer", leaving it nice and ambiguously open as to who it was. Steven vaguely mentions "all the trainers who give [him] tough battles", not specifying who. If no Memory Link is used in B2W2, N just talks aimlessly about "that trainer" as well.
Sometimes less is more, and I firmly believe that this is a case in point. As interesting as it is that the BW protagonist remains absent in B2W2, allegedly voyaging in a far-off region searching for N (and possibly getting into brand-new adventures) I don't expect or hope that we'll ever see it. It is far, far more interesting to have that be left unseen than it would be to show it - and it wouldn't really add anything to the overall Unova plot unless we ever get a third installment.