Why don't you write whatever you would like someone to write to you?
この場合、次のような言い方ができます。
ーWhy don't you write whatever you would like someone to write to you?
「あなたが人に書いてもらったら嬉しいことを書いたらどう？」
Why don't you...?「…してはどうですか？」
whatever you would like someone to write to you で「あなたが人に書いてもらって嬉しいこと」
ご参考まで！
Why don't you write down something you would be happy to hear?
"書かれて嬉しいことを書いたら？"
- Why don't you write down something you would be happy to hear?
- Why not write something you would want them to write for your birthday card?
- How about you write something that you would be happy to get on a birthday card?
カナダ 
アメリカ合衆国 
