Why change the damage of Fireball? It's pretty easy, really. The spell has been doing the same 1d6 per caster level since...well forever! Outside of when it kept scaling past level 10, the spell has barely changed over decades.
And uh, I just want to point out, monster hit points have increased dramatically while player damage really hasn't. Swords still do a d8. Ability score bonuses have gotten weaker over the years (remember when an 18*00 gave you +6 damage? Or when players could have scores above 20 without magic items?).
Fireball does 8d6 (8-48 damage) to a Challenge Rating 5 Troll in 5e. A Troll has 84 hit points.
Fireball does 5d6 (5-30 damage) to a Troll in 1e. A Troll has 33 hit points.
I would say, if anything, Fireball should do more than 8d6 damage to keep up with monster hit point inflation.