Fundamental Spellcasting (5e Variant Rule)
From D&D Wiki
This variant rule is intended for campaigns where the GM's want full-casters to have more variety in their Cantrips otherwise at low levels, because those that took a "flavor cantrip" will tend to keep repeatedly spamming the same damage Cantrip, which gets boring after some time. Or campaigns where you want more of a focus on the role-playing aspect without having the full-casters give up one of the few Cantrips slots that they have, to prevent the previous.
The "flavor cantrip" are minor magical tricks that novice spellcasters use for practice and thus, these classes should know it innately. Starting at level 1, full-casters know their class's "flavor cantrip" right away without counting against their number of cantrips known:
- Wizards, Sorcerers, Bards: Prestidigitation
- Clerics: Thaumaturgy
- Druids: Druidcraft
- Warlocks: Prestidigitation, Thaumaturgy, or Druidcraft, depending on otherworldly patron or choice. It is ultimately up to the DM.
- Others: If you're using a Homebrew class, it is up to the DM to determine which of the three is most suitable.