The bug

Unlike many other projectiles which use 2D textures, dragon fireballs do not use a 3D model, and instead billboard the texture as if it were a particle. This results in visual inconsistencies with other projectiles and also forbids further customization via resource packs.

How to reproduce

1. /summon fireball ~ ~3 ~
2. /summon dragon_fireball ~ ~3 ~
3. Closely inspect both entities

Expected results

Both of these fireball projectiles would appear to have a sense of "thickness" to them, due to using an item's model.

Actual results

The normal fireball correctly has a thickness, as it uses an item model. However, the dragon fireball does not.

How to fix

A dedicated model could be added for dragon fireballs, under assets/minecraft/models/item, or preferably a new dedicated folder for entities assets/minecraft/models/entity.