Yeah, the ult was actually kind of the last thing to fall into place on the kit, and I think players on on point talking about how it's a little underwhelming. Lemmie explain.
TL:DR; We wanted to focus power on the P/W, so the R had to either be (relatively) weak or complicated. Went with an ult that's mechanically really helpful for the character, but thematically pretty lame.
For this kit, we wanted to focus as much on the Stars of his Passive and W as we could. We found that we had a lot of fun, novel gameplay in a simple mechanic, but that it needed to hit pretty hard to be satisfying. So, when it came time to pick an ult, we didn't have a lot of power left to spare.
Now, a Star Dragon should probably do something really, really flashy with his ultimate. Like, biggest-thing-in-the-game flashy. And we tried a half dozen different things trying to find it. The trick is, something big and flashy has to have a big impact to feel right, which means more power.
A straightforward powerful ult (Ziggs, Lux) spends a lot of a character's power budget in one place - not a good fit, since we already had a lot of Aurelion's power earmarked for his W.
The alternative is something like Nunu ult - powerful but conditional. You have to jump through hoops to make it work, which lowers its effective power without lowering its potential impact. Great! Let's do that!
That's when complexity budget (the cousin of power budget) comes in. In playtest, we found that Aurelion was really hard to play well. His damage depended on his positioning game, and for a champion with no in-combat mobility, that game took a lot of thought. Trying to also do crazy channeled ults or weird strategic things while managing that got really difficult fast. It also made him very feast-or-famine - either he was on point with W and whatever crazy Ult, or he wasn't able to pull off either and we useless.
After much gnashing of teeth (muuuch gnashing of teeth), I settled on the current ultimate more for game design reasons than for character design reasons - it had the mechanics the character wanted. Burst damage, check. Long-range damage, check. Added safety (so you can screw up that positioning game a little and not instantly die), check.
There's a little bit of character design in there, though. We're hitting the "breath weapon" trope in a way that's a little bit different - that's where the name comes from. It's not "**Breath **of Fire", it's "Voice" - a much more refined thing than breath - "of Light" - a substance with much more purity than mere fire.
In my headcanon, Aurelion holds the very fires of creation, the light that ignites suns, inside himself, and with a single spoken word can spark creation. Very "let there be light".
So yeah, this ult exists to round out the character, to let him adapt to more situations and provide utility for his team even when struggling or behind. Mechanically, I'm pretty happy with it. Thematically, I'm sad I couldn't figure out how to make something more epic work out.
Hope that answers your question, because I wrote a whole lot of words by accident >_>
