This one could still work even in light of Islands. Just switch up where Finn and Susan land.
>Baby Finn's raft drifts into an underground waterway, where he's found by the hyoomans
>Susan's ship crash-lands somewhere in Ooo; despite the ship's total destruction, she survives albeit with amnesia
>Found and nursed back to health by a family of bunnies, she finds herself in a world needing a strong champion, and hell, who's stronger than her?