What would be neat is a reminder for the OP of a thread that he did not follow up to. Often you have people asking a question, getting provided an idea for a solution and they never report back if it actually worked. Or they are asked for more details and never answer those question.
So a reminder per Notification or - even better - per PN for such a case would be really neat. Trouble is: How to automatically recognize such a situation. Possibly easy for "question type" forums - if no answer is marked as solution then it is considered still open. However, we do very rarely use this forum type.