A mushoku tensei fanfic idea could be a gender - swap of the main characters. For example, if Rudeus was a female, how would her relationships and adventures be different? Or a story where the demon race and human race have to work together to face a common threat that was previously unknown in the original story. Also, a fanfic about Rudeus traveling to a completely different continent not mentioned in the original work.
Hard to say. It could be on hiatus, but without concrete information, it's difficult to give a definite answer. Try following the mangaka's social media or the manga's official website for the latest.