I hardly know where to begin unraveling this.
First of all, who uses Blizzard 2 as a filler during UI phase post-Freeze? At UI3 our MP recovers quickly enough we can get effectively one cast out before capping -- kind of a big problem with Blizzard IV's place in single-target, there's hardly time for it -- and that's largely going to be Thunder 2/4 until you learn Foul. Hell, thanks to Flare scaling its cost to our current MP, we don't even need to wait to be capped on MP before swapping back in AF, which actually increases our burst.
The idea of Blizzard 2 otherwise retaining a place for "rapidly recovering MP post-death" is unfathomably niche, not to mention redundant within the kit. We already rez with enough MP to cast B3, heaven forbid we need even less reason to use LD.
If Flare was meant to be purely a "costly finishing move", why oh why does it give you AF3 long before you get Aspect Mastery? Refreshing AF is one thing, capping it in a scenario when you can't possibly gain any benefit at cap (especially since you're forced into Transposing for about 18 more levels) is quite another. Sure there's always Manafont, but that's still a 3 minute cooldown versus an ability you're otherwise going to use every 20-30 sec.
And let's be honest here, I would rather have a one-stop shop for swapping and dropping than add an even more niche Fire 3 clone (I mean hey, Fire 3 at least has Firestarter) to the job. Adding another button for a task we already can do doesn't create more nuance or complexity, it's just another button. If the idea is "symmetry" between phases, then I invite you to consider that within our AoE rotation, Freeze as a combo of Blizzard 3 and 4 (swaps and generates Umbral Hearts) is opposed to Flare as a combo of Fire 3 and 4 (swaps and filler).
"Can't"? Unless and until the devs get around to buffing F2 so it has a contemporary position, Flare is strictly better because it gives more PPS. If it wasn't "better" we wouldn't be ignoring F2 now, would we?As I’ve mentioned, you can’t have F2 simply upgrade into Flare because Flare isn’t strictly better - it casts more slowly and eats more MP.
At present, there is nothing keeping F2 and B2 from upgrading to Flare and Freeze -- as Kabooa said, they're just superior AoE options.
Yes, congratulations, you've managed to make your idea sound comparatively attractive by throwing mud on what we already have, we're talking in circles at this point.4.0s Freeze would blahblahblah