"As previously announced, we’ve been reducing champion prices on a fixed schedule, primarily based on champion release dates. With every third champion we release, an additional champion currently priced at 4800 IP or lower will be reduced to the next lowest tier, based on age and ease of play. Initial price reductions have already been made on champions who were released prior to the ones listed below.
Here are the details on our next set of scheduled price reductions. Please note that because these price reductions are preannounced, refunds will not be granted to players who purchase these champions prior to their price reductions.
For our 4th champion launched in 2013 (after Zac): Skarner will be reduced to 4800 IP / 880 RP and Warwick will be reduced to 450 IP / 260 RP
For our 5th champion launched in 2013: Talon will be reduced to 4800 IP / 880 RP
For our 6th champion launched in 2013: Riven will be reduced to 4800 IP / 880 RP
Additionally, all newly launched champions in 2013 will be reduced from their initial price of 7800 IP to 6300 IP after one week. Click here for more background on the pricing changes and how these champions were selected for reductions."
TL;DR : Next up is Skarner and Warwick, followed by Talon, followed by Riven.