(A) main lead (either male or female or even better both) is tired/growing tired of his/her original partner also during that time they meet someone (the other main lead) who somehow suit him/her more. (My Mister, On the Way to Airport. Nice Witch, One Spring Night)
OR
(B) thing are going normal for the main lead and his/her partner, then he/she meet someone and begin to question his/her love for their partner. ( Chugaku Nikki, Kemono no Narinai Watashitachi, or to an exteny Spring Day of My Life (its also fit category A)
OR
(C) (also) thing are going well for the main lead and his/partner, but then some ridiculous thing happen, they got separated (willingly/unwillingly), and now the main lead grow affection for someone else. (Another Miss Oh, Miracle That We Meet, Time (to an extent),
also that plot should run quite deep into the story not just in ep 1 or 2
appreciate your thoughtful suggestion