* Afterthought: Something thought of later.
* Forethought: Thinking ahead or planning in advance.
* Prethought: Thought of beforehand or planned in advance (less common).
* Misthought: A mistaken or wrong thought.
* Overthought: Thinking too much about something, often to the point of anxiety.
* Rethought: To think about something again, often to change your mind.
* Throughthought: Thoroughly considered or planned.
There are many other prefixes that could be used depending on the specific meaning you want to convey.