Hey, looks like a really sweet system you have going here. I haven't had a chance to read all the posts in this thread, but hopefully the first few pages have filled me in enough.
Here it goes:
Aegis of the Almighty
Levels: Three if you can
Description: The hero who casts this spell begins to channel, and while doing so, they absorb all damage they would otherwise take. This absorbed damage is added to a stored value which will be used once the channel finishes. After channeling for a set amount of time, the hero is able to move about again, but is no longer able to cast the Aegis of the Almighty spell. Instead, they gain a replacement ability that will Unleash the damage they took by dealing AOE damage close to the hero. This damage is based on the amount of damage the hero took while they were channeling. Once the hero Unleashes the absorbed damage on his enemies, the Aegis spell is put back on their toolbar, and is on cooldown for a while. The cooldown starts once Unleash is used.
Now, when I say the hero channels, I mean they act the same way as Malfurion or Tyrande acts when they cast starfall or tranquility: Stands still and casts unless explicitly ordered to do something else.
If the hero is stunned or moves early, they will immediately gain the replacement ability, but it will only do damage based on the damage they absorbed for the shorter amount of channel time.
I'm debating whether or not you should add a taunt ability to the channeling half of the spell, might make it too ridiculous. In the event that you do add a taunt effect, don't make it for the entire duration of the channel. Half of the channel time would probably suffice.
The replacement ability should be called Unleash, and I'm figuring it will replace Aegis on the hero's toolbar until it is used. It should be a pretty huge AOE, but it shouldn't be able to be cast very far from where the hero is. It should be similiar to Flamestrike as far as graphics go, but it should deal damage in 3 quick bursts rather than over a longer period of time.
Unleash must be cast within a set amount of time once the hero stops channeling, I'm figuring 1 minute for all level, maybe less. As the level of the spell increases, so should the time the hero channels, perhaps 3/6/9 seconds(less if there's a taunt effect included). Finally, the multiplier for the absorbed damage should increase as the spell level does, maybe 1.5x, 3x, 5x damage returned through Unleash.
Tooltip: Nothing special, just needs to accurately describe the spell (good luck with that).
GUI vs. JASS: I'm in the process of learning JASS, so JASS should be fine. Your choice though.
MUI: I don't know what that means. Multiple users? If so, then no. It would just be an added bonus if you could do it.
Obviously this spell is going to rape face. I hope you have fun. :)
And if you have any questions, be sure to ask, I'd love to clear up any misconceptions.