Absolutely amazing. I'd started to think you'd abandoned this story given the long hiatus(like with what happened with Terrigenesis and Dungeons and Deathtraps). Luckily, I'm glad to see you didn't.

I especially loved the nipple-fucking and the throat-fisting. Those were two fetishes I think you've only done once before in your MLP story and in The Interview. Something you might want to do with those two kinks, however, is have some kind of end goal for them. For example, with fisting Mel's pussy, the end goal is likely to eventually get to her ovaries and eggs. For her anus, it's to either go all the way through her or prolapse her rectum entirely. With nipple and throat fucking, that goal isn't as clear. For nipple fucking, you could easily do something with breast milk and milk ducts/glands. Perhaps have Vicky try and milk her for cereal. That or you could somehow figure out a way to prolapse her nipples and/or figure out a way for Vicky to get to Mel's heart.

Throat fucking is a tad more complex due to its similarities with anal prolapse. You might be able to make something out of Mel swallowing something and Vicky having to reach down to her stomach to get it out. Bonus points for Vicky getting stuck and Mel essentially prolapsing her throat/stomach as a result.

You could also probably make use of Mel's navel as an entrance to her body and go about removing some of Mel's bones and perhaps organs from there. Her pelvis could be removed to improve her stretchiness. You could also have Vicky play a prank on Mel by having her remove Mel's bladder and route her ureters directly to her urethra.

As for brainplay. That one's bound to be difficult with the rules you've set for this story. If you bend them some, however, you could perhaps have one of the girls gain access to some form of portal technology and/or healing nanites. Then you'd be able to do whatever you wanted without too much issue and playing with portals is always fun.

Can't wait for the next chapter for whatever story you two decide to work on next. =)