For me it all centers around the worship of the beauty of the female body, everything else builds around this central nucleus - the feeling of submission is tied to a form of adoration/veneration/celebration of female beauty. Humiliation, domination and control&punishment are culturally coded symbolic ways in which this awe, respect, adoration is eroticized for me - everything to amplify the feeling of superiority of the female. This can take the form of trampling, worship (of not just feet), oral servitude, accepting of and catering to every whim expressed by the female, extending to more extremes like whipping, toilet slavery, etc.
Then there is strap-on and feminization, these two make no sense to me personally, since the first seems to find the female anatomy missing in any way (it seems like a stealth homoerotic feature - implying the old Freudian penis-envy theory), while sissification in my interpretation at least is deeply misogynistic, since it is almost always in the context of reducing the male into a female shape as a form of humiliation, which only makes sense in the context of the typical social devaluing of female bodies as well as the traditionally subservient sex-role females historically were relegated to. In my case, given the nucleus I was speaking of, it makes no sense, transforming into a female shape would be an elevation not a devaluation. Then from the heterosexual female dom's perspective, why would she wish to to see the representation of her own sex, as something inferior, as something humiliating? How could I respect and admire female beauty, if I were to see it as a lesser form? No wonder, that most sissification stories are completely fine with ending up making the MC serve males rather than females (again, it seems like something alluding to at least subconsciously homosexual impulses - no offence to homosexuals) Now, I may be in a minority, given all the sissification and/or strapon themes, but for me femdom is all about being a means to glorify, admire and submit to female power&beauty (both in its soft and extreme ways), which the latter two fetishes seem not to fit at all, but seem quite contradictory to it.
Sorry for the long rant