Huge fantasy of mine. Had the chance to play this out with an ex when we were young, dumb and... (I guess it's better to regret things you have done than things you haven't done.)
Would love to see my current partner get tickled by a voracious fetishist. Sometimes when I've tickled her I'll admit I've fantasised about someone else taking my place, someone merciless with a passing observation of safewords - almost like a horror movie villain except he has fingers and feathers rather than machetes and chainsaws
That story about your ex sounds like a good story