You know what they say. With great power comes great risk of that power rebelling and turning you into literal toast.
I definitely want to include special relics of some kind to grant Celebi and Hoopa the power they need to undo the spell-- I have to ideas. first, perhaps there are objects similar in vein to the Time Gears that can be used to give Celebi and Hoopa power equivalent to Dialga's and Palkia's. Second idea, perhaps the Lustrous, Adamant, and Griseous Orbs were required for the spell, but shattered when it went wrong. The resulting Resilient Shards, Glimmering Shards, and Twisted Shards would need to be collected and reformed to try and repair the damage.
Hmm... The Lake Trio had the power to control Dialga or Palkia in the gen IV games, a power Giratina possesses too... Perhaps the triplets could be the lieutenants? If so... I have ideas for some pretty interesting battles with them~ >:3