Well as others have said, extreme is to a degree in the eye of the beholder, still there are some general features of extremity which should be obvious from even an outsider perspective (just an ad hoc draft):
Prologue (in my opinion): genuine femdom concentrates on the female, thus forced bisexuality or sissification is not inherently femdom. The latter use the dominant female to accomplish repressed homoerotic desires, since their endgoal is not focused on the female sex, female body, female sexual needs.
1. The relational dynamics
a. Freedom vs slavery. How low is the sub situated in relation to females? Does she have absolute power over him? Picture this being on a spectrum, from almost equal but not quite (soft, playful femdom) to complete control and obedience (extreme femdom).
b. Permanence vs casual. Is the relationship femdom-like 24/7 or only on special occasions like in bed? The more ubiquitous the femdom is in the relationship, the more extreme it is.
c. Playful/with consent vs serious/non-consensual. Is the unequal relationship just a little bit of harmless fun, or is it entirely predicated on this imbalance, a relationship which hinges on this arrangement?
2. Cruelty.
a. From the dominant female perspective. What is the level of sadistic desire/intent. Does the dominant female enjoy being cruel/inconsiderate/harsh or is it only a consensual performance? The more she is truly a sadist, the more creative she will be in inventing and performing painful and or humiliating acts on the submissive.
b. Is the partner in the submissive role submissive, that is welcoming, even accommodating (in worst case demanding) of being dominated or is he/she reluctant? The more reluctant the one in submissive role is, the more extreme it is. Story-wise one can play with this to either progress from welcoming to reluctant, the other way around, or keep steadily on one disposition.
c. Consistency/rule based or whimsical. Think of it like this: is the punishment deserved due to some rule-braking or are there no definite rules when the sub gets a kick in the face just because she felt like it. Its obvious which is more extreme.
d. blackmail, emotional and/or social
3. Fetishes
The fetishes any bdsm relationship (femdom is simply female led bdsm with special characteristics) has are varied, not all of them has the same degree/intensity/meaning for everyone.
But as a rule of thumb: fetishes are symbolically heavy and are very much informed by the surrounding vanilla culture's attitudes and concepts of value and dis-value. For example, utterances which reflect power imbalances can vary across cultures. Acts which have profound meanings of being under someone (note the very concept of being subordinate) in one culture, may not have the same meaning or no meaning at all. For example a funny example for me is how in Chinese culture the victor forcing the defeated to crawl under his or her half-rised legs is seen as utterly humiliating, something which seen from a western pov may rather seem like nothing really.
Whats important is that all forms of fetishes obey the category the meaning of which they carry to illustrate the necessary power imbalance and unequal worth:
a. worship can be extreme if it gets to an almost religious admiration/veneration level
b. humiliation/degradation: a slap may be totally devastating in public and being laughed at generally can increase the embarrassment, but being used as less than a fully equal being has many expressions at its disposal - important here is not only what the sub goes through, but the ATTITUDE of the dominant party, the latter can make all the difference (or indifference)
c. related to humiliation/degradation and even worship is the exploitation of disgust, hence we have fetishes like licking muddy/dirty boots (not only literally below, but also disgustingly so) spitting, watersports, being reduced to a full toilet (scat) - these, especially the last one is almost self-explanatorily degrading, the one subjected to these has to overcome the universal disgust factor, the more disgusting being obviously the more extreme (note that this can also be interpreted from the sub's perspective as a signifier of accepted low status and level of devotion, after all if you see someone as beautiful and as desirable as to accept their bodily excretions into your body can be interpreted as total adoration to a point of a total subsumption of the ego, an act of truly and utterly giving up, or from the dom's perspective an act of utter and complete control/victory).
d. pain/punishment - this is self explanatory: the more painful, the more extreme. From just some slaps or spanking to being tied down and whipped severely to random beat-downs.
Finally I want to point out some fetishes which can be soft or extreme, but which are severely underused:
1. ponyplay - the dominant riding on the back of the sub, its instantly defines who is in charge innit?
2. facestanding/trampling - related to different poses of being above someone else, its a culturally intensive picture to convey this message
3. financial exploitation - we all know it hurts (its a good plot tool as well, establishing both a cause: debt, and a motivation to accept a role
4. female supremacy - my favorite, and its the extreme celebration and glorification of the female sex
I stop here, because I may never stop if not here
))
