Riot Games may have revealed a ton of new games they are working on during the 10th League of Legends anniversary but they seem to be far from stopping. Just recently, Greg "Ghostcrawler" Street announced Riot Forge, a new label that will publish singleplayer third-party LoL games.
Ghostcrawler first mentioned Riot Forge during a live stream but the was up soon after. According to the description there, this will be Riot Games' publishing label that will exclusively work with third-party developers who will work on story-driven, singleplayer video games in League of Legends universe.
Considering the games announced at the 10th-anniversary stream are of Riot's own making and multiplayer titles, this means we are in for a ton of other League of Legends content. Arguably the spiciest bit is that these games will be story-driven and that they will expand the LoL lore, which is already rich from all the champion backgrounds, the interactions of their regions and interactions between champions themselves.
Riot also put emphasis on focusing on both players and developers. The latter was elaborated on in one answer to some of the frequently asked questions. Namely, Riot Games will empower the developers that partner up with them through providing QA, localisation, voice-overs, research, marketing and more.
One important note is that Riot are still looking for developers to partner up with, which means some games are still several years away from seeing the light of day. On the flip side, one of them seems to be far into the development cycle and it will be , on 13 December 2019.
As for the platforms, Riot stated they are looking to bring these games to PC, consoles and mobile. The last one may bring some suspicion to potential fans but keep in mind they didn't state all the games will have to be on mobile as well.