On the League of Legends test server, client slots for skins for Fiora and Zilean have been updated. Such changes often signal the preparation of new skins and precede their official announcement. This was reported by insider Big Bad Bear.

There are no finished models yet, but both champions will likely receive skins in one of the three active lines: Nightbringer/Dawnbringer, Spa Day, or the new Variety collection. All three series have already received visual files on the PBE.

What Else to Expect in Future Patches
In upcoming updates, the following skins are expected:

Nightbringer/Dawnbringer: Yuumi (legendary skin), Kalista, Hecarim.
Spa Day: Ahri (legendary skin), Yunara, Volibear, Teemo, Sona, Sett, prestige Aphelios.
Variety: unconventional skins with original or humorous visual designs.

The exact release date is still unknown, but activity on the test server indicates that one of these lines may be released soon. Riot Games has not officially commented on the situation.