Considering making a FaerieTale AU
Where the player messes up and tells Flowey their name, meaning he has power over them, and they can't leave the "underground", or in this case, fae wilds, without Flowey just taking control of their body and piloting them back.
Perhaps the Barrier is a large ring of Rowan trees instead of an actual magical barrier?
Instead of collecting human souls, they're literally just kidnapping/holding human children hostage in order to bribe the rest of the humans into cutting down the Rowan trees. Perhaps Frisk could even get to meet them alive and healthy during their travels through the fae wilds, but can't really do anything to help them because they're all trapped due to fae magic.
The Dreemurr family could be deer instead of goats, just for the added whimsy.
Buttercups are replaced with Foxgloves, as they're more closely associated with fae while still being poisonous.
The Golden Flowers could be replaced with Forget-Me-Nots, because how funny would it be if I turned Flowey into a type of flower that represents love?
I'll think up more in a little bit, I gotta go do something.