Firstly this guide at any tame can be made obsolete and incorrect by Patch Updates. Current as of Alpha 0.4.8.1 Build.
The purpose behind this post, is to theory craft the optimal skill tree path for Fireball as with the current tree available and the level 20 cap. This means we have 20 points to allocate into the passive tree fore Fireball.
AOE: Area of Effect.
Single Target: Damage against one single Enemy.
Blasting Flames 1 Point [No Requirement]: Fireballs explodes on Destruction.
 One point wonder, causes all Fireballs to explode on impact causing AOE damage.
 This does effect the enemy which takes the initial impact of the Fireball.
 Fireballs that pierce, seem to apply this effect on each pierce and not just their final explosion. [Not sure if intended, need a Dev to confirm].
Lower Left Tree:
Piercing Heat 5 Points [No Requirement]: 10% chance per point to be double damage.
 5 Points is an overall %50 increase to damage. Any Fireball that hits has a 1/2 chance to hit twice as hard. This can then critically hit. EG: Standard Hit 10. Critical Hit 20. Piercing Heat Hit 20. Piercing Heat Critical Hit 40.
Frostburn 5 Points [Requires 1 Point, Piercing Heat]: 25% more damage per point against chilled enemies.
 If enemies are chilled from an alternate source (at this stage Fireball can not chill) this more then doubles your damage. EG: Standard Hit 10. Critical Hit 20. Frostburn Hit 22.5. Frostburn Critical Hit 45.
“Piercing Heat 2” 5 Points [Requires 2 Points, Piercing Heat]: Pierce and traditional enemy per point.
 Allows the Fireball to pass through the enemy and continue on is trajectory. This is not a single target damage increase, but does allow the Fireball to gain some AOE damage.
Spread of Destruction 4 Points [Requires 4 Points, Piercing Heat]: 25% chance to deal damage around an enemy on hit per point.
 Chance to deal additional AOE damage on impact. Appears to be 2550% of initial impact from my limited testing and will damage the enemy the initial projectile hits. Appears to be allowed to proc on projectiles that have pierced, but will not proc on the same enemy twice. Will be a large increase to AOE and decent to Single target, though the additional points invested in Piercing Heat if you not using Seeker’s Ash might be better utilized elsewhere.
Dancing Fire 3 Points [Requires 3 Points, Piercing Heat]: +2 Fireball Projectiles, 15% reduced cast speed per point.

Huge increase to your damage. With no other additional projectiles form your build, the first point triples your damage to multiple targets at a maximum assuming each Fireball hits. However if we take Seeker’s Ash and Ember, then this is also a large increase to single target DPS.

Math with Seeker’s Ash and Embers for percentage damage compared to prior skill level damage output.
[ ( N1 + 1 ) / N1 ] = Damage output compared to prior level. N1 = Current number of projectiles.
To elaborate, due to Ember halving the quantity of additional projectiles, each level of Dancing Fire will only add 1 additional projectile. To get the Embers node we already went through Arcane Divergence so we have 1 (2 but halved due to Ember) additional projectile, so we start the equation with having 2 Projectiles.
Level 1: [ ( 2 + 1 ) / 2 ] = 150%
Level 2: [ ( 3 + 1 ) / 3 ] = 133.3%
Level 3: [ ( 4 + 1 ) / 4 ] = 125%
So from this we can see each additional point is less valuable then the previous point, this is even more so if any points are allocated into Torrent [ I will revisit the math further on after Torrent’s own Math ].
Seeker’s Ash 1 Point [Requires 4 Points, Piercing Heat]: Fireball’s follow your target, removes extra projectiles unless you have Ember’s.
 Causes Fireballs to follow moving targets, multiple will NOT focus one single target. If the projectiles pierce then they will swirl around the target but not attack the same target again, but only cause final damage due to Blasting Flames explosions.
Rapid Flames 5 Points [No Requirement]: 6% increased cast speed per point.
 Pretty simple, max 30% increased cast speed, sounds great but we get a lot from equipment at this stage and a lot from talent tree so it has a lower damage increase compared to other nodes.
Incinerate 4 Points [Requires 3 Points, Rapid Flames]: 12% increased cast speed and 25% increased mana cost per point.
 Simple again, max 48% increased cast speed for 100% increased mana cost.
Plasmaball 7 Points [Requires 2 points, Incinerate]: 20% base damage added as lightning damage per point and 4% reduced cast speed.
 With 7 points, adds 140% additional damage (which scales from Critical Hit, Piercing Heat, Additional Projectiles) at the cost of %28 reduced cast speed. To acquire this node you gained 24% increased cast speed minimum so it almost evens out at a 50% increased mana cost. This is a huge damage increase more than doubling damage output.
Arcane Divergence 1 Point [Requires 4 points, Rapid Flames]: + 2 Fireball Projectiles, 100% increased mana cost.
 Big damage increase, big mana cost. Well worth it if we can keep up the mana regeneration. Again with no other additional projectiles this triples our aoe damage output.
Embers 1 Point [Requires 1 point Arcane, Divergence]: Projectiles fire in a line instead of AOE cone, halves the number of additional projectiles.
 Instead of your projectiles firing in a wide arc, they fire in a single line with only one activation of the spell. Though this reduces projectile numbers, it is required if you want to play with Seeker’s Ash.
Torrent 4 Points [Requires 1 point, Embers]: + 1 Fireball Projectile, 7% reduced damage and 40% increased mana cost per point. Projectiles not effected by Embers’.
 Additional projectiles at reduced damage on every projectile, at an increased mana cost. Let’s math it out:
To get to Torrent we must pass through Arcane Divergence so we gain 1 additional projectile and 1 per Torrent point.
[ ( N1 + N2 ) * ( 1  ( 0.07 * N2 ) ] * 100% = Total % output from all projectiles compared without any torrent points.
N1 = Current number of projectiles excluding Torrent. N2 = Torrent Level.
The following will assume for no points in Dancing Flame. Meaning prior to Torrent with 2 projectiles we do 200% damage (100% from each projectile).
Level 1: [ ( 2 + 1 ) * ( 1  ( 0.07 * 1 ) ] = 279%
Level 2: [ ( 3 + 1 ) * ( 1  ( 0.07 * 2 ) ] = 344%
Level 3: [ ( 4 + 1 ) * ( 1  ( 0.07 * 3 ) ] = 395%
Level 4: [ ( 5 + 1 ) * ( 1  ( 0.07 * 4 ) ] = 432%
What we can conclude from this is for a 160% increase to mana, we can more then double our damage output.
Now the math assuming 1 point in Dancing Flame, so 1 additional projectile after Embers effect. Meaning prior to Torrent with 3 projectiles we do 300% damage (100% from each projectile).
Level 1: [ ( 3 + 1 ) * ( 1  ( 0.07 * 1 ) ] = 372%
Level 2: [ ( 4 + 1 ) * ( 1  ( 0.07 * 2 ) ] = 430%
Level 3: [ ( 5 + 1 ) * ( 1  ( 0.07 * 3 ) ] = 474%
Level 4: [ ( 6 + 1 ) * ( 1  ( 0.07 * 4 ) ] = 504%
Now the math assuming 2 points in Dancing Flame, so 2 additional projectiles after Embers effect. Meaning prior to Torrent with 4 projectiles we do 400% damage (100% from each projectile).
Level 1: [ ( 4 + 1 ) * ( 1  ( 0.07 * 1 ) ] = 465%
Level 2: [ ( 5 + 1 ) * ( 1  ( 0.07 * 2 ) ] = 516%
Level 3: [ ( 6 + 1 ) * ( 1  ( 0.07 * 3 ) ] = 553%
Level 4: [ ( 7 + 1 ) * ( 1  ( 0.07 * 4 ) ] = 576%
Now the math assuming 3 points in Dancing Flame, so 3 additional projectiles after Embers effect. Meaning prior to Torrent with 5 projectiles we do 500% damage (100% from each projectile).
Level 1: [ ( 5 + 1 ) * ( 1  ( 0.07 * 1 ) ] = 558%
Level 2: [ ( 6 + 1 ) * ( 1  ( 0.07 * 2 ) ] = 602%
Level 3: [ ( 7 + 1 ) * ( 1  ( 0.07 * 3 ) ] = 632%
Level 4: [ ( 8 + 1 ) * ( 1  ( 0.07 * 4 ) ] = 648%
So what does this wall of text mean? Maxing Torrent results in an increase in DPS in all circumstances, to compare them with maxed Torrent; 0 points in Dancing Flames is 116% Increase (just over double damage), 1 point in Dancing Flames is 68% Increase, 2 points in Dancing Flames is 44% Increase, 3 points in Dancing Flames is 29.6% Increase.
Burning Flames 4 Points [No Requirement]: 20% chance to ignite per point.
 Chance on hit to ignite per point, maximum 80%, it seems that each projectile is an individual hit if using Embers or Seeker’s Ash, so in theory as it works currently we would only use 1 or 2 points in this skill just to gain access to the following nodes.
Still to do:
Finish explanations for northern grid
Find scaling on
Potential builds to math out:
Single target vs aoe / still using seekers ash for aoe / mixed single target
Maxed Torrent, Maxed Plasmaball, Blasting Flames.