Here's two I found. The Path Toward Heaven: Lots of mysteries, unique protagonist unlike other xanxia and xuanhuan MC's, characters and enemies actually have IQ. There may be romance, it's still early stages in the novel. The author is still writing it. The spiritual enlightenment of minghe: Strong from the start mc, more like a chinese epic on the birth of the world and mankind which is quite interesting. Completed. You should stop reading when he enters the battlefield for demons and gods because it goes downhill as in it's rushed, no new fleshed out or interesting characters and the ending is basically what would have happened over 1000 chapters in a single chapter. There's also no romance, that's either a plus or minus depending on the person or mood. The real funny thing is though the most powerful people in the novel, sages and others are all Arch-Wizards Spoiler Millions of years old virgins
My recommendation The Record of Unusual Creatures - little bit boring at start, awesome comedy, irony jokes, psycho goddesses, wacky tenants and sometimes heartwarming family moment.
Seven Unfortunate Lifetimes and Utsuro no Hako to Zero no Maria. The former is short while the latter is a JP LN predating NU, which limits exposure. But both are awesome reads and fully translated, so you should try them. And of course DenYuuDen (see my sig)
Not sure if it’s that unknown but a really good novel I recommend is Kusuriya no Hitorigoto! The MC has a bit of a obsession with medicine, but that makes her really adorable~ The characters are really funny as well!
I am not sure if the light novels I am going to recommend are unknown but they are not popular light novel series from what I know. My recommendations are Hundred and Absolute Duo. Both are great light novel series that are seriously underrated. A review of Absolute Duo can be found in my signature since the first and eighteenth post of my Absolute Duo novel pick up thread can be considered a review of the series because they are very detailed.
https://www.novelupdates.com/series/ore-no-osananajimi-wa-joshikousei-de-yuusha/ Yandere girl scams his love interest in become the hero in another world to realise her fantasies. https://www.novelupdates.com/series/dont-read-this-novel/ Tried this one by chance, even with only chapters released so far it was good. Reminded me of those 90s scary programs for young adults. Also these, not sure if they count as relatively unknown tho but here you go: https://www.novelupdates.com/series/shini-yasui-koshaku-reijo-to-nana-ri-no-kikoshi/ Isekai with female protagonist who is more oblivious than any shounen mc and able to find more trouble than the Rocket Team itself. Troubles that she solves in some way or another. https://www.novelupdates.com/series...-villainess-but-why-did-it-turn-out-this-way/ Coolness. This protagonist seems made with that single trait, Spoiler if she is tasked to partecipate in a battle she ends winning the war, if she is invited to a relative house she ends saving the life of a newborn, if she is tasked to guard a noble girl she ends making the protagonist of the original timeline fall for her, etc, etc She becomes so cool that along the way she forgets how to girl lol. Basically what would happen if The Rose of Versailles and the Goblin Slayer series have a child.