A slippery slope is not always a fallacy.
To claim this is a fallacy, there needs to be a clear distinction between master pyraminx and the other puzzles that we're saying might also be added. How is master pyraminx different from gigaminx or teraminx or 8x8x8, such that the slippery slope is truly a fallacy?
From a standpoint of popularity of puzzles, I'm not sure. I own two gigaminxes and an 8x8x8, but I don't own a master pyraminx, so from my very bad sampling of one cuber, I'd say the master pyraminx is less popular than gigaminx or 8x8x8.

But in general if there were numbers that indicated master pyraminx was a particularly popular puzzle, I'd consider that a good argument for possibly adding it.
Master pyraminx is more practical in terms of solve times, so that could be a justification. But there are plenty of other puzzles that also have "practical" solve times.
I think it might be nice to add a couple of new puzzles next year, but I would like them to be selected based on criteria that give good justification for the choice. If there are one or two non-WCA puzzles that are clearly more popular among cubers than others, I'd see that as a potentially good justification.
I also wonder a little about mirror blocks, even if it is a puzzle that has been around for ages. We recently had a solver who is fully blind join our forum a couple of months ago, and mirror blocks is one puzzle that could truly "level the playing field" for blind solvers, especially if we made the event "mirror blocks blind, no inspection". Just a thought.
I'd like to have some discussion over possible events for next year, then have a poll (with choices based on the discussion) to see how many people claim they would attempt to compete in the event every week. Then I might select the top one or two, if they got enough votes, and add them to next year's list of events.
Note that I already intend to change Fewest Moves to a mean of 3 next year, since so many WCA competitions now use mean of 3 as the format for that event. (I need to start preparing for that change soon - it will be complicated making that change to the website and maintaining compatibility with old results, so I'll need some time to get it right.)