I'd much prefer M/f (or F/m) to F/f - F/f leaves me cold. But that's just me.
As for ideas: Tickle machines. An android or artificial intelligence as the 'lee. A retelling of the Andromeda myth, where Andromeda is tickled half to death by the sea monster before being rescued. An interrogation story using tickling *but* where tickling is *not* a torment - the victim is made into a tickle addict and spills the beans for her next fix; the victim is tickled to reduce her resistance to telepathy, truth serum, or the mind probe machine; or some other clever twist allows tickling to be useful as something other than a torment.
Interrogation where the victim is forced to be the 'ler - the 'lee is the interrogator, and can (read the victims mind/tell if the victim is lying/divine if the victim is a witch) while being tickled.
Contests between 'lers - who is the best tickler? - with a 'lee (or more than one) as the judge. Dueling, with the duel 'fought' by tickling, rather than with swords or pistols. Tickle boots (or shoes or sandals). Tickling as a secret weight-loss technique on a fat farm.
A special celebration or holiday, where the public is invited to tickle the feet of the Sultan's harem, while the rest of the harem remains hidden behind the harem walls.
Tickle slaves, where it's the 'lers who are enslaved, and forced to provide tickle-services to the elite 'lees. (Maybe the tickling drains energy/life force/magic power from the 'lers and charges up the 'lees.)
A 'lee who is desperately trying to learn to like being tickled, the way everyone else does (or the way the nobility / sorceresses / upper-class wealthy elite do).
A sorceress, as part of a cunning plan, deliberately botches a demon-summoning, so that the demon tickles her silly. You'll have to come up with the cunning plan yourself; if I knew what it was, I'd write that story.