It looks like the Game Rating and Administration Committee of Korea has rated the currently unannounced Sonic Origins Plus.
As spotted by Gematsu, the Korean rating board has rated what is probably an updated version of 2022’s Sonic Origins, now with the added Plus tacked on the end of its name. It doesn’t look like the rating is currently viewable on the site itself, but as noted by Eurogamer, downloading the CSV file listed does list Sonic Origins Plus in Korean–written as 소닉 오리진스 플러스의.
Obviously with only having a rating to go off of, there’s no way of knowing when we can expect a potential reveal, or even what the new content could possibly be. It is possible that Sonic Origins Plus will function similarly to Sonic Mania Plus. First launched in 2017, Sonic Mania later received a Plus version in 2018 which included two extra playable characters, a new game mode, and expanded the competition mode, all of which was available as DLC for the original game, just compiled into one package.