So I'm back here in this thread now that I have an actual idea of what the "accessory" would be, if these games contain any.
FRLG had a wireless adapter, because the original games were basically THE reason to have a GameBoy link cable in the first place.
HGSS came with a PokéWalker, because there was a similar pedometer-like device for gen 2 games, which had a Pikachu, and walking gave you points that you could exchange for items.
So both accessories had something to do with accessories that were used during the original games' release. So what did gen 3 have?
The E-Reader.
"But Rivvon," you say, "No one used the E-Reader! And we have AR Cards now! Why would they make an accessory of that?" To which I respond: "First, who used that pedometer from gen2? And second, you're right."
I'm placing my bets now on AR Cards. Cards that unlock various Trainers to battle in-game, like the E-Reader cards of old. Maybe the cards we get are randomized, so it would encourage players to trade the cards. Maybe the TCG would do something alongside it? It's hard to say on that front, but my guess is AR Cards at least, if anything.