The unique feature is its setting in another world. It allows for a lot of creative world - building and the introduction of strange and interesting creatures and cultures.
Well, without having read the 'milf hunter from another world novel' specifically, we can make some guesses. Maybe the protagonist, the milf hunter, travels to this other world in search of something valuable. Along the way, he encounters various female characters who are'milfs' and gets involved in their stories. It could also be about him trying to solve a mystery or a conflict in this other world that is somehow related to these women.
The titular milf hunter is surely a main character. He could be a complex character with his own motives and flaws. There may be other characters such as local inhabitants of the other world who play significant roles. Some of the milf characters could be main characters too, for example, a milf who has some sort of power or knowledge that the hunter needs. There might also be a character who is in conflict with the hunter over his'milf - hunting' activities, perhaps a protector of the milf characters or someone with a different view on how to deal with them.
Well, one common theme is the power - up of the main character. In many such fanfictions, Issei would gain new abilities or powers as he goes through different situations in the story. Another theme is the complex relationships within the harem. Each member of the harem has their own personality and backstory, which leads to various kinds of interactions and conflicts among them.
Another one could be 'Alice's Adventures in Wonderland'. It takes the protagonist to a strange and magical world, much like the concept of 'Another World Milf Hunter Novel' where there is exploration of an unusual world. However, it doesn't have the same 'hunter' aspect but has its own unique charm.
Issei develops by facing various challenges. Each encounter in the harem and with the milf - hunting situations makes him grow. He becomes more confident over time.
No, Boruto is alive in the manga. The plot hasn't led to his death. However, the manga is ongoing, and anything could happen in the later developments.
One similar novel could be 'Reincarnated in a Fantasy World'. It also has elements of a character exploring a new world, though the focus might be different. Another option is 'Adventurer in a Strange Realm' which shares the concept of being in an other - world setting.