Is it permissible for me to think of another man while still being married, but contemplating divorce, as long as I do not participate in any inappropriate behavior with him?
When you are still married, you are not allowed to think of another man at all. Even talking on phone or messaging him or any way of contact is not allowed. After you get your Islamic divorce and pass the Idda period after Talaq, then you will be allowed to do that.
Wassalam.