Lots of possibilities come to mind.
Being a falsely accused victim on Law and Order or The Practice and getting tickling revenge on the assistant district attorney prosecuting the case, be it Law and Order's Elisabeth Rohm or The Practice's Lara Flynn Boyle (Helen Gamble). For that matter, in a few more recent episodes of The Practice, Rebecca Demornay made some guest appearances (still lokking very hot). She played a bitchy lawyer who needed to be "put in her place and I would be happy to help out.
Going back a few years (okay more than a few), being a powerful Genie on I Dream of Jeannie or a powerful witch on Bewitched and tickle torturing Barbara Eden (Jeannie) or Elizabeth Montgomery (Samantha Stevens) for all their screw ups sounds like fun.
Being a "bad guy" on Charlie's Angels and tickling a stubborn Jacqulyn Smith (Kelly Garrett) to try and get some information sounds like fun. She would hold out for a long time forcing me to continue the torture for a long time. Naturally, she would eventually fail and give me the information (hehe).
Still, I think the greatest "fantasy" is one that has been writen about numerous times here. To be a guest star on Batman, needing to tame the very lovely Julie Newmar playing Catwoman. For years that's been an image stuck in my head. Still hasn't happened, but one can continue to hope. That's why we have re-runs anyway, isn't it?