I think Morrowseer's coming back because he'd probably help Fierceteeth and MIndreader stir up a rebellion against Glory. You know, like saying that the rainwings don't care about them because they would actually let a fellow member of their Nightwing tribe to die (Morrowseer and Mastermind, and since it was stated many times that they could never let a member of their tribe die, no matter how odd they are), causing doubt to arise amongst the Nightwings on whether Glory is actually following through on not killing them and letting them live in the rainforest. I also think that the Nightwings are still unsure of the Rainwing's intentions in letting them stay. A bit of dialogue between the captive Sunny and Morrowseer would also be interesting. Like her asking him why he made up the prophecy and her questioning if her whole life really was a lie.
I also think Whirlpool will replace Morrowseer's place in planning with Blister.
Sunny's most likely broken from learning the prophecy was a lie, because all of her life she dreamed of saving dragons, and prophecies most likely end up true, so she thought that for certain they will save Pyrrhia. A little hope in the times of sadness and war. But since the prophecy is fake, the 100% chance of saving Pyrrhia is gone, now it's a tiny chance. The dragonets were never meant to be heroes, they were never meant to save Pyrrhia. Now Sunny is believing she really is useless, because she has no purpose in her life anymore. (Poor Sunny)
A bit on the optimistic/super unlikely side, Morrowseer might actually make a truce with the dragonets, since the Nightwings are living in the rainforest now, and now he has no reason to side with Blister.
I support Starspeaker at the moment, but that might change in the fifth book. Since Starflight and Sunny are both kept away from each other. Maybe Starflight will reassure her that she really isn't useless and that they will make the prophecy happen, whether it was fake or not, and Sunny will comfort him in his blindness. (Daaawww)
Morrowseer was most likely not killed by the fireball because because they considered saving Viper by pulling her out immediately. Which leads to me thinking that if dragons are only exposed to fire/lava for a brief moment, they'd still be alive. Vengeance was also alive for a while after he got thrown into the lava. Morrowseer would only be exposed to the fireball for a brief moment. But if lava consumed him, he would be drowning in it, killing him. But he didn't get exposed to lava, so yay. Unless the fireball was many times hotter than lava. (I love lectures :D)