Possibilities are endless, but you're probably going to want to avoid unintelligent monsters, which would be against the bestiality rules. That leaves you with intelligent monsters - not a great handicap - and there are some interesting thoughts to develop on that line. For example:
Perhaps one half of the isle is dominated by male creatures (centaurs, minotaurs, satyrs, etc.), while the other half of the island is dominated by their female counterparts. The males have been driven rapaciously insane, so the females keep them contained...and the women themselves have begun to be affected by whatever lust infects them, but with a lesbian twist.
Or the isle is the abode of a sorcerer or sorceress, who has summoned strange creatures from different worlds and dimensions, who each have their own unique ways to feed and breed...and Kate must watch herself, lest she become pregnant, or transformed to better suit the creature's lusts. Along the way, she draws closer and closer to the center of the aisle, and the sorcerer/sorceress themselves...
The imps, terrible as they were, did Kate a favor; she now smells like demon spunk, which wards off certain of the island's predators...but may attract others.
Aside from traditional mythical creatures (centaurs, sphinxes, dragons, etc.) you could get creative with nontraditional mythical creatures like the
blemmyes or the
panotti (getting head from a blemmye would be no joke!) Alternately, you could create something original, like giant ants that want Kate to become their new queen (ovoposition ahoy), or a female vampire that needs to feed off of something other than blood.
Such scenes might be made more interesting by a choice of scenery - select ruins, a copse of trees which were once men and women (and parts of them still are), a cave, a pool that has some magical effect (bigger body parts are often a favorite, but shrinking, aging and de-aging, etc. are all fun - maybe there are two fountains, and one has a curse and the other has a cure).
A plot to stumble across would be fun - maybe something simple like find the (broken?) fertility idol that has attracted all the creatures/drives them mad with lust and try to harness its power. If Kate stumbles upon a piece of the idol, it might explain why the inhabitants are targeting her.
