It's about whatever pairing do you want (my example in BaekYeol) but the plot is..

Baekhyun always wrote what happened in his days in a diary, so when Chanyeol wakes up he can read what ever Baekhyun wrote. Baekhyun one time wrote that he wanted to suicide but decide to keep it off from everyone except the diary (so then Chanyeol can read it). After some times passed, Baekhyun begin to wrote lies in the diary, writing that he is fine, when in fact, Baekhyun has been doing some drug use and some family problems appeared. When Chanyeol woke up, he found the diary in the table beside his hospital bed, he reads it and decide to check on Baekhyun. After Chanyeol was released from the hospital, he ask people about Baekhyum and found out the truth, that Baekhyun was in a rehab, but then in the middle of rehab, Baekhyun decide to suicide.

Bad grammar haha..

Hello! Truths, Lies and Secrets. Hmm.
What do you think about this? It's about Xiuchen/Chenmin

Xiumin was too truthful. He did everything to make Chen happy. He would confess if he did something wrong and he did not dare to do anything that would make Chen upset or jealous.
Chen was lying. From the beginning, he did not love Xiumin. He only accepted to be his lover because he pitied the boy and he gets a lot of good things from Xiumin.
Something that was made out of Xiumin's love and Chen's pleasure was born. And it was meant to be kept a secret to everyone. Their baby had to be locked away from the outside world to forever remain a secret.

So yeah, Mpreg (Because I love those things). Chen and Xiumin are married and yeah, Xiumin thought Chen loves him very much but he concluded wrong. Chen wanted their baby to not be brought outside as he didn't want Suho, the angelic man Chen loves, to see the baby. So yeah, it's an angsty one.

I'm all about YUNJAE!!!! Wooooo

I think you should stick to something more unnatural. I'm tired of cliche storylines which you can predict already. It's like kdrama.

Theme: unsaid truth, lies, and secrets.

Well Yunjae seems to hold lots of secret. It's how you'll find out the secrets. Like some secrets are meant to kept away never revealed but sometimes someone finds out. My idea: Jaejoong has lived happily with his life. He's a pure, innocent and kind person who wouldn't hesitate to help anyone. But when he meets Yunho, life wasn't what he thought it was. The truths he was told were lies. Yunho was the key to unveil a past Jaejoong had forgotten and his family kept it a secret from him. Telling him lies that he believed. Now he struggles to find the real truth in the web of lies.

Since I like baekyeol so I prefer you write about that one kkkk
But their image doesn't really suitable about the theme... Truth, lies, secret, seemed a bit serious, baekyeol image was cute and I think if u want to take the theme, kaisoo is the most suitable (maybe?)
It depend on what type of writing you are the best at? If it about high schooler gang or so, with bit of tense of relationship or fighting mode, maybe u can write about the unpredicted boy with thousand of secret and just showing his though image that has a relationship with someone, has so many obstacle during the togetherness and there is a big reversal in the end would be good.
Or if u really like sad-romance-classic story u can make one person having serious sickness, have to lies till almost the end of the life to protect someone he loves.
Or maybe hiding true identities is interesting too... Not knowing that someone he date is royal prince or somewhat like that? Kkk. It's going a little bit fun I guess, moreover if it is about baekyeol, can't imagine chanyeol is royal prince from somewhat kingdom and pretend to be average netizen for getting closer to the poor kid named baekhyun kkk.
*just giving my best to help ^^; sorry if it a bit distracting

Hmmm....truth, lies, secrets....
Trying to think of something original.... spies and affairs seem too obvious...
What about witness protection...one is in protection but meets someone they really like when they are relocated. But then something happens and they have to be relocated again. How do they explain it? If they tell the other person whats going on they could be in danger.
Truth, Lies, Secrets....
idk....

What about the most cliche thing on earth like the boy is actually sicks but kept it a secret from the girl.

Wich pairing? ?
What about Baekyeol? Hmmm. A naive Baek in love with Channy, that's just passing time while his lover (maybe Soo) is away doing God knows what in China?
Of course Baekkie is oblivious to everything and just go deeper and deeper, to find about everything in a very sad and tragic way.
lolololol~
You can have truths, lies and secrets with that! (I suppose)
I'm not a writer, ok? It's just that I'd love to read someting like that ^_^