Except it means you can only use it in clean-field situations.
I'd consider it, at level 3, to be a serious downgrade over the default "surgical" fireball. If your allies are fighting a bunch of monsters and maintain any formation, the surgical one lets you blast almost all of the monsters.
I guess evocation wizards, who can sculpt spells, get a big boost.
As well will Sorcerers. They have Empowered spell which would allow them to reroll dice to have more control over the radius, as well as Careful spell in case it does end up too big. Empowered doesn't even need to be used until after the damage is rolled, and can be used in conjusction with other megamagic.
Oh you can add things like Dragon Sorcerer +CHR to damage
and radius if fire is your thing.
But yeah, evocation wizards get a
huge boost from this. Others will have to use it in the traditional way of getting it off before the party and the foes intermix.
So, that's the two full casters that get fireball natively on their spell lists.
At higher levels, fireball is far outclassed by later spells anyhow.
Meteor Swarm is 40' radius times 4 for 40d6 damage. This upgraded fireball at level 9 does 14d6 in an average of 50' radius. Night and day weaker.
True, at 9th level there's a better spell. But fireball stays competitive at a lot of slot levels.
Starting at 3rd, it upcasts to this average rolled damage:
| 28.0 | 31.5 | 35.0 | 38.5 | 42.0 | 45.5 | 49.0 |
Only a single 4th level AoE does more damage at any level, Vitrolic Sphere. But it has a save that reduces it a lot more than a firball save-for-half so it's expected average damage is lower.
Going up to 5th level, Cone of Cold beats it by a point. Slightly more when you upcast them both both 5th.
No 6th level AOEs do as much as a 6th level fireball.
It's not until 7th level that the spells start doing more than fireball. Which is more than double the level you got fireball, and also after when most campaign end according to the D&D Beyond data and WotC survey response they've published.
(Damage per level source for spells:
)
So a fireball that starts with a 28' radius and grows bigger outclasses many levels of spells. If you're playing a standard 1 to 10-ish game, fireball is basically always top tier AoE for every level it can be cast at.
Barring very unusual situations, you don't run into 500 orcs running at you in tight formation, which is what the insane higher radius on higher level fireballs needs to actually scale faster than linearly. The larger radius will get in the way more often than it won't. And when it doesn't get in the way, it is very fun.
You're the one saying you need density, that's almost the opposite of what I'm saying where the extra area is important. Please don't try to set up a strawman. It could be a field of hobgoblins all nicely social distancing with an empty square between them, and if there are enuogh targets the extra area will get them. Again, there are times when just having an extra line or two of squares or not an be very useful.
And you are ignoring that you pick if an how much to upcast it. So we can basically assume that it will be bigger when it's advantagous to be bigger and smaller when it's advantageous to be more controlled.
To sum up: A 20' radius fireball is top tier for damage at every spell slot level a normal caster will reach (5th-12th level). Making it default to about twice the area at 3rd level (20'r = 1256 sq ft, 28'r = 2463 sq ft), and giving the caster more control over making it large selectively by upcasting makes it even more powerful than it already is.
Cast a 9th level fireball, roll 14d6 and get an average radius of 49 feet. BOOM!
