Riot Games used to release new champions like there is no tomorrow but the growing roster made them slow down. Almost our months after the last release, we didn't have any new teasers but apparently three will come during the 10th-anniversary celebration.
Qiyana was the last champion Riot Games released and it happened on 28 June 2019. Following her release, the developers went radio silent about the champion roadmap, not divulging new information on upcoming champions, including the previously announced AD Carry.
That all stands to change on Wednesday, 16 October 2019 as Riot Games will release the new champion roadmap that includes information on reworks in progress and three new champions. With that said, we advise you not to get too hyped as this is a roadmap, not an announcement.
In other words, the three champions' release dates will most likely be spaced out over several months so don't expect them all to come out together as part of the 10-year celebration. Riot did not elaborate further on the upcoming champions.
On the topic of Volibear and FIddlesticks reworks, however, there is more information but that's due to the voting program results. Namely, Riot let players vote on a group of champions in the earlier half of 2019, in order to figure out who the players want Riot to rework the most.
Players had a choice between Dr. Mundo, Volibear, Shyvana, Fiddlesticks and Nocturne and could get one rework out of the five. Since Volibear and Fiddlesticks came in so close, Riot decided to go for both champions and the recent announcement probably means we are getting the first teasers on Wednesday.