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Before I forget it: let me express, just how much I enjoy this comment you delivered here.
I am sorry; it took me so long, to respond here.

Well, this does create some intriguing possibilities.

Clearly opens up a few doors, to very interesting (intreaging) posibilities here. Now I just hope you will be enjoying the options I'm putting into play for this story.

Having the Spa be a covert love-gathering station for changelings presents a number of questions.

This could be the key, to an interesting story, wouldn't you think?

Are the Spa Ponies collaborating with a Hive, or have they been replaced at some point?

Doesn't appear to be the case. Hope this is not dissapointing.

Since the replacement seems to be pretty short-term, is this an ongoing project,

This particular aspect would be a pro case basis.

For that matter, could this actually be something sanctioned by the Princesses after negotiations?

I guess they could be sanctioning it openly, after they realize, go there and possibly some negotiation as well.

if the harvester keeps things to one-day "visits", I could see most of the Princesses taking a rotating schedule of visits themselves,

I guess the Princeses could be very interested in taking part in this, at some point. A day, or a weekend free from the oficial duties sounds like a fair and very interestingdeal; wouldn't you say?

just to make sure the process is safe for their subjects.

The Princesses are very keen on securing the safety of their subjects, as far as I can see.

As a pilot project, I could even see that sort of spa becoming a chain of locations across Equestria.

This could be a very promising futurte, for both Ponies and Changelings, aside from anyone else interested intrying this treatment. And whatever other arreatments may be developed in the future.

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shouldnt you rate fetish content as mature?

Who, or what gave you that idea?

11927835

Being fetish isn't an automatic Mature,

Exactly

but this story is definitely doing things too intense for Teen.

Could you please explain, and elaborate on this?

Ps: I have several stories under the "Fetish" tag,
which was Approved by the Moderators at the FiM site as is.
Just to clarify the situation, in case someone is not aware.

Rarity is obviously enjoying her slow introduction to submission. An now she's got a take-home accessory, probably the first of many to come. I'm definitely enjoying this slow burn, though now you have me curious about how many other clients might be getting the same service. It's not as if the Spa couldn't support multiple ponies as things expand in the future, after all.