Riot Games unveiled the use for Sett's calling card, tied to the champion who is joining League of Legends' roster on 14 January 2020 and continued by teasing the other two champions that are headed for release in the same year.
Sett's Calling Card was earned in the weeks leading up to Sett's release. Players had to get First Blood 10 times in order to acquire it and Riot Games finally revealed what the use for the item will be. The card will unlock Sett for free to anyone who earned it which is quite handy for those saving up their blue essence for the next time the Essence Emporium shows up.
Meanwhile, there are also two new champions on the horizon. One of them is a jungler who will once again be themed around trees or flowers as it is said to blossom. Given that this is the first tease, it's not unheard of that the rest of the details are more than obscure.
As for the second one, it appears this champion will have some sort of invulnerability mechanic or at least a post-mortem one, akin to Karthus or Sion. Riot's actual wording states that the champion is a "masked stranger who refuses to die". While a mask might hint at a new Assassin, keep in mind that League of Legends introduced masked champions in other roles. For example, Jhin may be a murderous maniac but he does not fill the role of an Assassin gameplay-wise.
These two are the only ones Riot Games teased but they also noted there will be more champions in 2020. Considering that the statement is in the plural, it's safe to assume 2020 will be more eventful than 2018 where we got only Kai'Sa, Neeko and Pyke.
Speaking of events, the teased due will be tied to a "major in-game event" that will happen during Summer 2020.