On the other hand. I think this sort of thing should have been done as far back as 2E so it wouldn't have been an issue in 3E.Monte Cook: "Fireball is a static 5d6. If you want more damage, you
use a higher-level spell slot."
Unless hit points are scaled back dramatically, this is a REALLY bad idea. Fireballs in 3e were useless, due to hit point inflation. I really hope I can ignore this rule without breaking the game.
In 1e/2e, a 5d6 fireball is useful for about 2 levels. Then it's useful against goblins and kobold slaves. That's it. It will do 8 points of damage on average if a save is made. So your average 3 hd creature is still up, wailing away on the party. I suppose the fighter can get an AOO on the ogre while it's helpless from laughter, though.
Firstly, fireball was designed to take out a lot of little creatures, not one big creature.
Because HD were 1d8 hp, the average creature in 1e/2e had 9 hp for every 2 HD (4.5 hp per HD). Since an average 5d6 fireball deals 17.5 damage, it can outright kill 4HD creatures with only a slightly above average roll. That's an entire swarm of carrion crawlers (3d8+1 hp), dead. 4 HD creatures are hardly guaranteed a successful save, after all. For 2 HD creatures it's even worse, because they probably won't survive even if they make the save!
Sure, if a save is made it's a lot less damage, but are you actually telling me that you think fireball should kill creatures even if they make their save!? That's about as silly as suggesting that a fighter should kill creatures regardless of whether he hits or misses with a basic attack.
Lastly, 5e fireball is only capped to 5d6 based on the spell slot you use. Presumably, if you use a 4th level spell slot (assuming the 5d6 uses a 3rd level slot) it deals 7d6. So you can either use a higher spell slot and be able to wipe out scores of "6 HD" creatures, or you can use the lower spell slot and use it for handling lower level minion-type creatures, virtually guaranteed to be numerous when you encounter them.
Because burning a 4th level spell for one or two d6 is almost always a bad decision. 2d6 damage but I lost my only charm monster or polymorph spell is not a wise use of spells.
I'm not assuming anything. Like I siad in the op, we don't have enough info for that yet. But the tidbit dropped seems to be a bad idea, because it either nerfs fireball "to the ground" or it's an entire shift in mechanics to fix something that ain't broken.
Agreed.
Sure, I alluded to the same. I may have missed it's usefulness by a hit die or two. Fireball should be useful creatures a lot higher than 4 hd.
A 6 hd creature has a 35% chance of making it's save. Even if it fails, it's still up.
I never said that. I'd just like to see it useful long after level 7 or so.
Because burning a 4th level spell for one or two d6 is almost always a bad decision. 2d6 damage but I lost my only charm monster or polymorph spell is not a wise use of spells.
Stop putting words in my mouth. Pre 3e, fireballs, lightning bolts, etc were staples of every mage and useful even at very high levels. Capping them at 5d6 will make them tools of the vermin exterminator, nothing else.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.