Cirno
Nothing is confirmed, but the Christmas skin will definitely come back. It may not be the exact same, but it might, but it'll come back
As for summer, it's hard to say. But with the international children's day event. There's TONS of new skins coming back. Including one of my favorites, the gummy bear!
Welcome to the server! Let me know if you need any help getting achievements. I'm glad to be of help to you.