According to Pokebeach, then next pokemon movie (after the Arceus one) will possibly feature Ho-oh and Lugia. As to what extend the two will play is unknown, but I'm not really that surprised as Ho-oh has yet to have a major role in any movies.
One question I have is will Entei, Raikou, and Suicune also play a part as they are a major part of Ho-oh's history. Only Entei and Suicune have been in movies so far, and of the two Entei was they only one to a have a really important role. Suicune, although in a movie (Pokemon 4ever) it only appeared late in the movie to help Ash fight and late save Celebi. Raikou has never been featured in a movie, only in a two part special. Another is does this mean that Ash & Co. will be returning to Johto or will this movie be taking place in Sinnoh? It would seem odd for a movie featuring the two major Johto legendaries to be anywhere be in Johto, but who knows. They will probably introduce yet another anime only town that somehow has some connection to the two. I'm rooting for Johto personally.
In other related news, a friend of mine mentioned to me the other day that he plans on buying a japanese version of HeartGold, and I'm considering getting SoulSilver. Once he (or we) get our versions I'll certainly post any new areas and information.