@Mandy I like your way of thinking. Love your ideas of her being lonely on her birthday or Valentine's Day. Maybe even Christmas because it's considered a couples holiday in Korea. Tan of course would still be faithful to her and feel pangs of sadness and longing when her realizes he's been neglecting ES, but he should find little ways of making it up to her.
As for Tan's constant jealousy over any guy that so much looks at ES I think I can include some scenes for that. I don't think a major love triangle though, but maybe some minor flirting from some men or a small crush from one of ES's colleague's at work/school.
Young Do definitely needs to become ES's friend again. They got along really well after he stopped bullying her. I thought about possible girlfriends for him again and was thinking maybe a new character can be introduced. Possibly one of ES's co-workers at the broadcasting office or an old classmate from her old high school or a classmate from her college where she is taking her masters.
You think there should be any physical fighting or scuffles included in "Heirs 2?" Since there was so much violence present in Heirs 1" I thought maybe I should tie it in some how.
Writing is complicated lol