Pokémon Platinum game director Junichi Masuda told What They Play, "At this point, we're not thinking of going in that direction."
"Trading is a core concept of Pokémon," Masuda said. "So when you're trading, you meet with a friend and decide which one you want and which one they want. I would like to emphasize real-world communication. You don't see each other online." This statement paints Diamond and Pearl's online trading, then, as some kind of concession rather than a defining feature.
Masuda also said that the concept for Diamond and Pearl was to produce the "ultimate" Pokémon game. For Platinum? "My direction to (game designer Takechi Kawachimaru, also present at this interview) with Platinum was to produce another ultimate version."