Is naming things a fantasy in Japan? In a lot of isekai manga, the main character comes across someone or something (e.g. a slime monster) that has no name, and must give a name to him/her/it.  Every time, the one who is given a name accepts it, is happy, etc.

For example:


In Tensei Shitara Slime Datta Ken, Rimuru names all his villagers
In Tondemo Skill de Isekai Hourou Meshi, ch. 10, the main character has to give a name to a newly tamed slime
In The New Gate, Shin (ch. 10) and Schnee (ch. 26) give names to newly tamed beasts


Since isekai manga kind of aim to make true all the dreams/fantasies of the reader (e.g. the main character is super strong and has successful romance), why is there such a focus on giving names?
I think you meant to say "fetish" rather than "fantasy"?

Not that I'm aware of, but in many cultures, naming is of great significance, there are often attitudes that being given a name or title is perceived as a great honour. 

Catholics does this, for example, when they receive their baptized name; as well as Buddhists, when they receive their dharma name. 

Many monarchies have a tradition of the monarchs taking on a new regnal name when the new monarch ascended to the throne.

It's also common trope for feudal servants/subordinates serving a new master to take a new name, it's a sign of throwing away their past and therefore subservience to the new master.

Another common trope is a new pet being drawn to feel happy when they're first named. Often the trope goes that the pet feels unhappy/angry/uncomfortable while the owner scrolled through a few names that then got rejected before they ended with the one that the pet seems to respond well to (IRL, this is likely just reflection of the owner being happy for having finally picked a name, because chances are the pet doesn't care or understand).

This is definitely not unique to Isekai genre.