I have almost never played Frost during Legion, therefore i have little to say about the spec. It has been pretty dominant throughout the whole expansion, but people always called it stale and too much proc dependant, so i can see why the removal of Ebonbolt (which guarantees a proc and adds into the gameplay) can be seen as an issue. I will be curious to see what will be done about that.
Fire, imho, has a whole chapter of improvements that can be done. Namely, talents. Cinderstorm can be reworked so that it does not pull mobs in China while standing in New York and so that it focuses all cinders on your target. It could also guarantee an Heating Up or a whole Hot Streak, so that it melds better into the rotation, and i would increase its cd to 30 secs: this will greatly improve its viability as a competitive talent in that tier.
As @
Obelisk Kai said, Phoenix Flames and Flame On in the same row poses a problem that cannot be ignored during design. Being forced into a decision between 2 things that feel both mandatory is not a good feeling. Flame On should become baseline and in its place it would be better to have a niche talent that you can use occasionally, leaving Phoenix Flames as the obvious choice. It made the spec so much better than sticking to it would be most beneficial.
Arcane has been my go to, and i like how it works. Mechanically, i think they should make it so Mastery becomes more relevant. Savant revolves around many aspects of the spec's core, damage on charges aswell as mana regeneration (which should be the most important part of it): yet, it has become less relevant the more gear we got to the point where it is the worst stat by quite some margin, which makes very little sense. I am glad to see that they already changed one aspect that needed to, Arcane Missiles procs. They are so important to our burst phases that, expecially in the opener, bad rng would cause our mightiest burst (due to pre-pot, trinket procs and usually BL) to feel much less potent that it should. By making Missiles castable at will we can work around mana consumption or simply deem it worthy and just nuclearize our victim, which will be awsome (although i am pretty concerned into the PvP balancing of a completely on demand damage burst like this).
Like i have been lenghtly discussing before, AoE should be finally disconnected by its melee reliance, because it's an old system that will always mess with the spec's viability at some point, regardless of how much or how importantly. A talent that makes Missiles splash their damage on everyone around our target (maybe replacing that useless Unstable Magic), a rework of Nether Tempest so that it pulses AoE when struck (which, again, would synergize pretty well with our on demand Missiles) or a new channeled spell that creates a sort of static arcane orb on the target or in a targeted area that pulses damage on everyone and gives a charge each time it deals damage (which would be definetly fitting for Arcane, since basically no spec bases their AoE on a proper channeled spell anymore). There are many possibilities.
In general, i think Mages need a pretty hefty rework of their talents. Rune of Power should be hacked from the game, its code deleted, destroyed, infected and sent to hell alongside Windows Vista. Incanter's Flow, which is often ignored thanks to its proximity to the devil, should follow him to hell. It is the most boring and less sense making talent in the game: oh, look, my spells do more damage now! And now they do less damage. And now they do more again!. Why? What's the logic behind it? Just bake its damage into the class and move along. Next in line is Unstable Magic. Too unreliable to have any value in AoE scenarios, which leaves it as a ST increase only because nothing else does in that same raw. Zero sense. Scrap it, add some other stuff.
I am not a huge fan of Mirror Images as a dps cd. I think they are better suited for the content where people already make use of them, which is Solo/World content, where the Mage usually has a harder time than most other classes and it could use a nice boost. Therefore, i would expand on the theme: drop the images and call it "Illusory Guardian", a nice good tanking pet on a 2 minutes cd that lasts for 40 seconds. It would make a world of difference with Elite killing and in many other aspects of the game where us Mages feel mainly useless.