If you stay in the bath forever, everyone else will get out and you'll be here all by yourself.
You better wake up, otherwise we'll leave without you.
この場合、次のような言い方ができますよ。
ーIf you stay in the bath forever, everyone else will get out and you'll be here all by yourself.
「いつまでもお風呂に入っていると、みんな出て、一人になっちゃうよ」
you'll be here all by yourself. で「ここで一人になっちゃうよ」と言えます。
ーYou better wake up, otherwise we'll leave without you.
「起きないと置いて行っちゃうよ」
we'll leave without you. で「置いて行っちゃうよ」と言えます。
ご参考まで!