Our beloved fire mage sure did change for the worse,thanks to the removal of Bolt of the Storm and Flamethrower.
This guide aims to provide a workaround for those who still want to bet on his damage potential.
First of all,the rework made Living Bomb pretty much a non-viable choice for almost ANY situation.I know it will take time to adapt but it's good to remind that the standard DWW combo is not viable anymore,unless early game.
Early game on KT is now the key to his success as completing one of the two given quest talents on level 1 will decide how hard it will be later on,especially when reaching level 10.

Flamestrike was nerfed by a good amount,compensated by a level 1 talent
Living Bomb now has base Chain Bomb and a 10% range increase but can't be forced to explode by casting it on the same person.Also no minion spread to heroes
Gravity Lapse had some talents changed(a.k.a indirect nerf through the 30% speed removal)

Only if you are willing to spend early game spamming Q in order to max this talent.It will prove useful,especially against standard teams.Avoid if facing a dive happy comp.

the 5% added bonus is just for tickling opponents.The healing amount provides a laughable amount of healing,even considering the relatively low CD on Verdant Spheres

The safest pick at this point,versatile and solid.The barrier was nerfed again but may still prove useful later on.Good against dive happy/tank compositions

With speed nerf this talent is pretty much useless.Avoid

A pretty average talent,as there's already a talent further on that decreases CD.

The one and only pick at this tier,especially if you picked Convection at level 1

Good luck lining them up.Might be useful againt a double tank comp but still a debatable talent at best

This could be the go-to talent in this level if it only wasn't for the slow CC that comes with KT's kit.A decent pick if going double Verdant Spheres

Another candidate for must-pick talent on this tier.Good luck getting close enough to land it without getting smoked.In short,a situational pick.It does give more overall damage than Gravity Crush extra bonus to Flamestrike though

No nerf will replace Phoenix as the standard pick for virtually ANY build.

Paired with Arcane Power might turn into a hassle for ranged assassins.Even if they have a mean to escape it still keeps them busy.Unfortunately it goes both ways Also,holy cooldown batman

The 90% of the time go to talent on this tier,as the others vary from are either plain useless to rather situational.It helps to extend your combos and that alone is the reason to consider it

A pretty much useless talent,since Living Bomb can't be triggered to explode anymore.The slow amount is also ridiculous

This may semms like a nerf but i'd like to remind that Living Bomb radius had a 10% increase. I know some people might think that this paired with Master of Flames may be good combo but it needs too much CC in order to succeed.

No longer the goal Tier of the match for any Kael'thas player,it still provides a decent amount of talent diversity

With the cooldown cut in half this might be a viable choice,especially against melee comps.Requires a lot of effort though

As with Sunfire Enchantment good luck getting bla bla bla.Might be useful in a 1v1 situation(al).Then again,weren't you supposed to be on the back line?

Allows to setup many combos.Also paired with Mana Tap becomes a never worry-about-mana-again button. Using it to empower your E will increase its stun duration by 50%.Not a game changer increase yet still welcome,paired with the possibility to hit multiple targets. There's a catch though:your E is still slow as ever,so timing needs to be right.No,it's not a panic button.

Situational pick good for maps like Cursed Hollow and such.Otherwise useless

What?who?really?

Might be the best choice if your team provides infinite stuns,or your faith in humanity is huge.

Straight damage increase,like a timed version of Gathering Power.If you picked double verdant as 16 talent the mana restore is useless anyway.The 60 secs CD may allow it to pair with Pyroblast,like once every 2 casts.Otherwise requires timing and bla bla bla