Bandai Namco on the official Twitter channel of the Idolmaster: Million Live! Theater Days announced a new feature from the game. This title going to use 3D graphics like the previous IMAS smartphone games for example the The Idolmaster: Cinderella Girls Starlight Stage.
But according to this message players can change the 3D gameplay mode to 2D which makes the game easier to run on mid-level devices or older iPhones. In the message you can already see a 2D screenshot.
— ミリオンライブ！ シアターデイズ【公式】 (@imasml_theater) June 20, 2017
The new Idolmaster: Million Live! Theater Days will be available from 2017 in Japan on iOS and Android but the release date is still unknown.