Yumejoshi (夢女子 lit. dreaming girls) . A term which describes “dreaming” about being in certain series / media, and having typically romantic relationships with a certain character, or characters. Originating from, “dream novels” which was fan-fiction where you could change the heroine’s name to yours & experience being alongside your favourite characters. Think something like Y/N’s in fanfic. The male, or masc-aligned term is ’Yumedanshi’' (lit. dream boy), if memory serves me correctly.

This sort of thing is also called "Self-shipping", which is a term you’re probably more familiar with. But, I don’t think the term(s) Yumejoshi/Yumedanshi solely encompass self-shipping.

I’m not entirely informed on this topic, so I invite anyone more well-versed than I to chime in. If anyone who identifies with these terms follows me, feel free to comment.