In a 5e campaign where I played a halfling Warlock, and found that Fireball isn't on the basic Warlock spell list, I convinced my DM to allow me to create a Fireball-like spell, since at that point I didn't have a lot of choices for AoE attack spells, and my party-mates could all do more single-target damage than I could.

Since the 5e go-to attack cantrip for Warlocks is Eldritch Blast, I figured I might as well learn into it and called it Eldritch Boom. The effect is similar to a sonic boom. Instead of catching fire, creatures and objects in the AoE that fail their save are knocked over (creatures are knocked prone). For the damage, I conceded that I couldn't just copy Fireball, so instead of 8d6, I went for 6d8, but higher level slots add 1d8 per level.

The DM allowed it mainly because I was the main spellcaster. But later on, our party got a new addition: a pyromaniac sorcerer. Around that time, I switched to Blight as my go-to attack (when I didn't just use Eldritch Blast).