I was imagining in my head what would happen next in this drama.
My imagination was running wild as I thought about what would happen in the next scene/episode.
ご質問ありがとうございます。
① "I was imagining in my head"=「私は頭の中で想像していた」
"what would happen next in this drama."=「このドラマで次に何が起きるかを。」
② "My imagination was running wild"=「私の想像は暴走していた」
"as I thought about what would happen in the next scene/episode."=「次のシーン・エピソードで何が起きるか、と考えながら。」
ご参考までに。