The plan all along was to continue the story between Tian Jing Zhi(Janice) and Xue Ling Qiao(Kim Tae Hwan) if they could get the same cast and write up completely new characters if they couldn't. Also Kim Tae Hwan was probably never tapped for the sequel due to the hallyu ban while Janice Wu also had scheduling conflicts.
Source: http://www.dramapanda.com/2017/08/my-amazing-boyfriend-2-begins-filming.html