I really hope Hasumi's outfit in this event will be wearable
I hope so too, but realistically I doubt it, it just looks like the underlayers of her transformation to me, like the boss girl had just undressed her so there's nothing special to give reason to add it. Plus from twitter comments regarding the slave suit effect, it sounds like its neither simple nor quick to add a reskin to each sex animation in the game, which would be required to add this costume to general play. I would have expected them to have budgeted a week or two of dev time for that if it was going to be playable in the final version, and i haven't heard mention of them working on that at all.
I don't want to get my hopes too high for new stuff in the final version, given how little left there is of the game that we've yet too see:
What we will almost certainly get:
Boss for 3-2
Special event(s?) related to 3-2 boss, may just be first-time intro dialogue, might be more
3-3, with one new general enemy
3-4 Boss and GO
Gallery for viewing animations
What we might get:
GO scene for 3-2 boss
Additional erotic scenes in the base (maybe one regarding the slave suit?)
Additional hypno-charger machines in 3-3, expanding on the event surrounding them
Novel-format (text based?) hypnotic interrogation event for 3-2, mentioned in FAQs as something they'd like to add if they have time left this month (related to hypno-chargers?)
What we won't get:
A fourth world
A third plant stage
Additional stages or enemies post-release, confirmed in FAQs
At the end of the day, we have seen like 75% of the game already, there's a limit what what they'll have been able to add in the last two months.