The comments from Lady Grey, Miss Lily, and others have been invaluable to me in attempting to understand the long-run evolution of sadism in a relationship.
It would be safe to say that all of my relationships were with Dommes that were somewhere "along the learning curve." While I do believe that individuals will continue to evolve, change, and develop in their roles, I think it is different when the "end product" changes as opposed to people who have yet to experience the multitude of plateaus that will inevitably occur over time. To put it simply, a veteran who has done nearly everything she has wanted to try is significantly different from a novice who has dabbled but yet to experience the bulk of their fantasies and interests.
I have only encountered a handful of Dommes that I would call a "finished product." That is, they have an extremely strong sense of self and have enough experiences before their long-term loving relationship to have figured out exactly what fuels their Domspace. I do not know if this is always the case, but a few of them had quite a bit of non-romantic D/s interaction before performing it in a loving long-term relationship. These women are confident enough to have the "nope, doesn't do it for me" mentality towards activities and not because they do not understand the dynamics... they understand the dynamics just fine, it just isn't an interest. This can occur due to a thorough understanding of the mental dynamics or from first-hand experience.
Less experienced Dommes are "works in progress." They likely have fueled many ideas through fantasy but just have not had the experiences to clearly define their limits and boundaries. Many ideas are intriguing, and assuming they can find something that taps into their Domspace, they are often wanting to try a lot of different things... to see what reaches them. These are the Dommes that often start slow but then progress at a frightening rate. In many cases, once they embrace the "lust for power" it becomes a major driving factor and occasionally the symbolism behind an activity will speak to them more strongly than the activity itself. e.g. "It makes me feel more powerful" or "It reinforces the D/s dynamics".
My first-hand experiences have taught me that left unchecked, the work in progress Domme can often just keep on pushing... going further and further until things just "don't feel right anymore." The scope and limits of their Dominance are often a result of hindsight rather than foresight.
In the blogging sphere, "Forever Hers" is a blog that comes to mind of this type. I'm guessing only a handful of readers will remember them (and I certainly hope I am recalling the correct blog here). It began as a male sub blog and within a few months both parties were blogging. After about a year she was the only one blogging and I got the distinct impression that he wasn't allowed on the internet anymore. Within a few more months we were getting stories of her taking him out in a pair of Daisy Dukes and sending him into the store with the assignment of buying Playgirl magazines. My first impression was "what the hell? Is this the same couple that was dipping toes into the water six months ago?"
Over time I've come to realize that my own experiences mirrored this. If I try to picture the mind of F (or K or T to lesser extents) I picture something along these lines playing on a loop in her Domspace:
"Oh, you could handle that. Now try this. Oh, you could handle that. Now try this." Every push farther... every shove deeper... and it got her more and more aroused at the symbolism of what she was doing with me. I was dragged along for the ride but managed to always adapt... and even come to crave this interplay. I could only imagine what she would become... and as of the last time we spoke she had multiple subs serving her, they were all kept naked except for collars and cuffs, and all locked in long-term chastity with zero orgasm privileges. She ruled over them with a whip. Oh, you could handle that. Now try this.
I'm really trying to wrap my mind around the contrasts and differences but a lot of things are becoming more clear. If F had the opportunity and safety to experiment over the course of her life... instead of allowing 20 years of fantasy to fester and burn... would she have turned out differently?
I know that my own submissive response to her "loop" is something along the lines of: "Yes, I can handle that. I love you even more. Yes, I can handle that. I love you even more." It is quite frightening to know that is the pattern of my inner-workings.
I also hope that this makes more sense in regards to where things have gone in fs01. In Arc 5 I started writing Cassandra as a "work in progress" and fs01 was getting dragged along for the ride knowing... Yes, I can handle that. I love you even more.