We have the duty to shill for Polish vtubers that are the only europeans based enough to not be hated by Nool
Or is just a april fools and they are finally going to make Kuro troon on streamCould be several things, new members being the most obvious "thing to come".
The picture is of all the current members, all using the default outfits so it's not announcing matching new looks or anything like that.
Also, most likely not a concert because I don't think everyone would be taking part and they wouldn't be using their default looks. Same with one of their group covers.
Then again, the date being April fools, I could imagine the shadows obscuring some stupid face or other quirk on their models they'll all have for the day, a likely option but a boring one.