A Couple points.
As to not having the "Right Frame of Refrence...Donny!" on the history of D&D.....I am 38 years old, and have been playing D&D sice I was about 6; rightly, wrongly, strictly by the rules as written, in every edition from basic to 4E....and frankly probably every edition to come next.
I have no interest in edition wars because there is always Another Edition.
If there is no edition beyond D&D Next, then I want D&D Next to be as good as possible because I am going to play it.
I love 4E monsters...what they became at least at the end. There were a lot of years of inflight tinkering on 4E monsters...but Monster Vault can hold its head up high in the pantheon of D&D monster books. Perhaps even higher than my prior generic monster compendium favorite, the 1993 2E AD&D Monstrous Manual.
That is gone now.
Scaling is an issue, but one could obviously point out.
1E/2E Fireball was probably too powerful, being relevant at very high level. This is in general true for Magic in general, and The Forgotten Realms in particular.
3E like most attack spells, Fireball fades into non use. The spell DC just keep up with the monsters. The damage progression was still d/6 per level, now with a cap on dice. With the increase of monster HP in 3E, the spell is designed to fade into non use. It quickly becomes apparent it is better to use the slot to buff the warrior to do some Power Attack/ Great Cleave , or Whirlwind attack, crowd control than cast Fireball.
The Attack spells that remain relevant at high level having factors that minimize being subject to AC, (Ranged Touch Attack), Spell Resistance (Conjuration School and the Orb Line), Did Not Allow for a Saving Throw, or attack ability scores (Con Drain), or the ability circumvent HP in general (Drown).....you know the Druid Spell list with Spell Compendium.
4E: The trend for Damage Dice expression is retained. Flaming Spehere takes its Greatest Form ever in the history of the game. The trend though is status effect and denying the enemy his actions, and or subverting them is more beneficial. Fireball is pathetically presented in this game, as the 7th level Enounter power after it is nearly as good.
You can account for the variance. I think it is a factual statement to say a Fireball from 1-3E would have a reasonable chance to kill a basic Orc out of the monter index outright. The exact details differ, but the general statement is true.
More specifically I will say this, I think a Fireball should do 30/15 damage. I think the save DC, should escalate to keep the spell relevant at high levels.
The spell should also have some status factor, a minor ongoing , easy to put out Fire damage and a knockdown effect.
I think your G.I Issue should start at 30 HP.