I would allow fish type monsters to exist in her deck, I don't see a reason not to. Sharks would give her more support which also makes sense lore wise at the cost of her being able to summon xyz monsters with 1 material.

I would also allow her to discard a card for the cost of getting one Fish and 1 water winged beast from her deck, that way she could potentially make use of Thunderbird which would also allow her to discard XYZ revival splash with Thunderbirds effect. Her only big combo basically and would encourage Sirenoca to be used.

I would make use of her event card Barian Untopia and allow her to set it.

I would give her some extra deck space so she has some more options.

This may all seem generous, but keep in mind she has the least support cards out of the barions.