Playing a melee Circle of Spores Druid and wanting to use that 1d6 extra damage I'd get if I cast Guardian of Nature and take the Primal Beast form. Here are the relevant texts from each spells:
"...For the duration, you can use your spellcasting ability instead of Strength for the attack and damage rolls of melee attacks using that weapon..."
Guardian of Nature
"Primal Beast. Bestial fur covers your body, your facial features become feral, and you gain the following benefits:
- Your walking speed increases by 10 feet.
- You gain darkvision with a range of 120 feet.
- You make Strength-based attack rolls with advantage.
- Your melee weapon attacks deal an extra 1d6 force damage on a hit."
I know the Great Tree form specified Wisdom-based attack rolls, but would it be out of the question if the benefits from the Primal Beast form would apply to melee weapon attacks despite under the effect of Shillelagh?