Yeah, if Celebi is held through an event dealing with the GS Ball, then Celebi's OT will definitely be you, the player. I think this is pretty decent as a lot of Pokémon we get through Wi-Fi are never ours, but TRU or something else. They'll definitely not stop events like said, as it beings in income for the company and promotes others to buy the games if they really want that event Pokémon. Not to mention, a Pokémon like Mew, Celebi, Jirachi, and even Shaymin won't be freely given so easily by Game Freak or Nintendo.
If they include the Sevii Islands, then so be it, but it shouldn't remain empty. I thought about it, and since what happened in FRLG is 3 years before Johto, then the plot with Team Rocket on the islands were already completed. So, either it'll be there, but empty, it'll be there with a new plot, or it won't be in at all. Whichever they choose I don't mind, because my main place they should keep together is Johto and Kanto. These two places made the Johto what they are and shouldn't be altered to me.