So far this is Countess specific event.
It is possible there will be others in the future. Tho with how pissed the majority of the community is about it, idk. Not that Plarium cares about that.
Bottom line is, if enough people buy Countess, they will likely make other champions purchasable with money.
They did this before with Hordin, albeit slightly differently as Hordin was available in the shop. It wasn't well received then and this event isn't being well recieved either. If they start putting champs like Bad El, Valk, Krisk etc up for sale this game will lose all value and a lot of players will quit. They must rethink this, as soon as you take away the element of chance the whole game is devalued and they will lose players as a result.