Hiya! ^-^
For some context - while in development, we always start with blue sky ideas - literally anything goes - and then prune the ideas into what we need, and what odds and ends we can add to a skin to really make it feel cohesive in terms of the thematic, i.e., should it be legendary, should it be 975?
Now, picture that, but we're also doing this with ~10 skins at a time, and needing to allocate resources (animators, audio engineers, etc.) appropriately! We plan pretty far in advance, so we know what's coming down the pipeline, so we can inform other teams (like publishing, or merch!)
We're always open to suggestions and feedback, but there are limitations.
Taking a lower tier skin and turning it into a legendary isn't an overnight process. It involves several disciplines, and several weeks worth of work, which weren't originally planned for. We'd be looking at several weeks for animation alone, in order to provide what legendary tiers should offer. Legendary skins also have new VO - this also means hiring said voice actress (and really, hoping they aren't booked!), recording, cutting and editing, implementing, etc.
TL;DR: It isn't in realm of scope for what was planned out. I hate being the bearer of bad news, really.
That said, there are things that can be within scope for a skin like this - particle changes, small model changes, texture updates, a new sound added. We do hear out feedback, and ultimately, the team working on the skin makes the call on what suggestions they feel enhance the skin as a whole. I know that the color change for Diana's outfit was something that was heard for feedback, and had a pretty quick turnaround on the PBE!
Ah, and then there's the toggle! Yes, we have done toggles in the past (Neon Strike Vi and Night Hunter Rengar, and somewhat with DJ Sona, but that's hooked up a bit differently). We may veer away from toggles because they aren't always the right choice for a skin, and we could do more noticeable (read: cooler) things with other aspects, whether it's animation, particles, etc.
It was decided while in concept that Blood Moon Diana wasn't going to have a toggle, but that we would show the mask removal in the recall animation, which is why, in the splash, you can see her taking the mask off. That's where the two make a connection.
New animations aren't something that are added once a skin hits PBE, especially when it was planned that it wouldn't be included in a skin. Why not? Animations can take a week or more (plus testing!), and in the case of a toggle, hooking up tech that a regular skin wouldn't have.
So, about postponing? As I'd mentioned before, we plan ahead so other teams - a big one is publishing - can come up with a release plan. Now, that can be anything from a simple announcement page, to a micro site and video, and even more. Removing a key piece of the publishing pieces could be a nightmare for publishing, who, at this point, has most of their project complete and ready to roll.
A good example here would be if we took last years Star Guardian patch and removed Lulu. This would deeply affect the promo pieces, especially the video and the survey that determined which Star Guardian you were.
Don't get me wrong, I totally see where you guys are coming from - Blood Moon Diana is a killer skin, and there's a lot of hype around her! It's awesome to see so much passion for a skin, honestly. Sure, I wish we could make every skin a legendary 'cause let's face it, legendary skins are an awesome opportunity to push an alternate fantasy. But, this time around, it isn't in the cards for Diana. That said, it means she still has the opportunity for a legendary skin in the future! Suggestions always welcome!
I hope this doesn't come off as a 'hey, here's a bunch of excuses'. but rather a bit of insight into the skins process and planning. :)