Make Black Kats have the same spawnrate as Mewkats or lower, after the event is over. Who knows, maybe we didn't get them all this time and there will still be some left over. Also, make them have a cooler entrance animation. For something that big and scary, it deserves more than a "poof!".
Keeping Black Kats as a possible spawn after the event is over
Make they spawn as rare monsters on arcade (where they can drop tokens). and even more rarer on Mission (where they dont drop tokens) so people can look for them in arcade in hope to make it a bit alive and have a bit of chance to get their kowls. they only drops page tokens but not the book. Book only will be a drop in the event time, so people can still have a small chance to get token and the book keeps rarer since its a event only drop. that can work even better and give a bit of "unique reward" from arcade in some way.
I have got a better idea why not make black kats permanent and even their boss who drops some new token.
Make them appear in a danger room. At least one should spawn behind a closed gate for most of a battle, only to come out at the end with however many Carnavons it churned out in that time.
I would love this. It's cool to see it once in a while specially when it's totally unexpected. I still think it should have token drop, but have it spawn a bit more commonly than rock jelly or something.
I hope they stay. Even if they don't drop tokens, books, or kat hats more mobs is always better! A better reason for curse defense outside of UFSC is also needed.