While it would be interesting for them to open the GS ball after all of these years, I doubt that they're actually going to do that. It's been eight years and they've never mentioned anything about it after Ash got his second Johto badge. Besides, as J!!! mentioned, the producer mentioned that they were originally going have Celebi come out of the GS ball, but they decided to do the fourth movie instead and hoped that their audience would forget it. I also believe that he said that they would never go back to solving the GS ball plot.
Filler episodes can be pretty enjoyable, but it depenRAB on their general plot within the episode. I also have a feeling that before Ash goes back to Johto, they'll do something with that island that you can go to after beating the game. I could be wrong, but I'm thinking that they'll do that for awhile and then go to Johto for awhile.
I think that it is a bit premature to worry about this now though since HeartGold and SoulSilver have only been confirmed for a few days and based on the episode listings from Serebii, Ash is just about to battle for his seventh Sinnoh League badge next week in Japan. I don't see Ash getting anywhere near Johto until somewhere this fall at the earliest, but it depenRAB on the pacing for the rest of the Sinnoh saga.