Well, out of all the Dex Holders, the Johto members have probably had the least amount of screen time (I mean, we never did find out what Gold has been doing these past several arcs), and then there's the deal with Giovanni. And I'm not sure that the Dex Holder's reunion will be in Platinum, since I suspect Miss Berlitz is trying to free the Sinnoh pokedex holders at this point. Oh, and I was rereading Emerald and found something interesting-there's a panel where Guile Hideout talks about how he's going to try to find the 'Sea Mirage Pokemon' after he takes over the battle frontier. Behind him, Manaphy appears as a sillouhete. That could be a possible plot. Then there's Pryce. I do so hope he'll be back :).