It's Mermay so here's a fun AU that came to me.
Who's seen The Swan Princess? Take that, TLM, and Tangled.
Instead of the tower, Gothel has some access to magic and turns Rapunzel into a mermaid and leaves her hidden in a cove/grotto/other during the day while at night she becomes full human? Her human half let's her still carry the hair for Gothel to use while the fish half ensures she never leaves.
Eugene can basically still find her the same way, except instead of the cliff falling near the tunnel, it falls into the cove, and you got Rapunzel saving a drowning Flynn Rider, only to still tie him up after and the whole deal or I dunno. This time you have the night/day shift to contend woth for the journey though.
It was just a random idea that came to me for the month and looking at all the mermaid images and being like 'but how would this work with the story?'