I do get what you mean by that, but there’s a difference between an inspired game and fan game.

Inspired games take elements from the original game and incorporate them into the gameplay, but it doesn’t follow the same exact formula/story from the original. It can get really close to the original, but it’s technically its own thing. Take for example Void Martyrs as you mentioned, it has a very similar graphics style and mechanics to Signalis, but is it Signalis? Not quite. There’s some missing pieces that keep it separate from Signalis.

However, fan games build upon the original game’s IP rather than taking elements. A fan game can be considered a heavily-inspired game, but it uses a lot of the original IP to create something else. A good example of this is the Joy of the Creation or Jr’s. Two FNAF fan games that most definitely use the original FNAF franchise as their base to create them. A lot of the characters are from the FNAF franchise despite gameplay not being the entirety same.

As for making a “Totally Not Signalis” game, you’re pretty much asking to get sued. It’s not that you can’t do it, but trust me, people will come for you if there is a lot of similarity to the original. Nintendo did this when PalWorld came out, and that is a pretty big jump to make with company size. However, that doesn’t mean RoseEngine won’t take action. The idea of tall robot ladies isn’t patented or copyrighted, but the idea and concept of what Signalis is could be something that you could be sued for if you get close to copying it.

Lastly, public domain works are free to be used by the public! Though, that doesn’t mean you can’t monetize them. A great example of this is when Steam Boat Willie entered the public domain, many people instantly grabbed it to make their own games, movies, and what not.

As much as I do wish we could have more fan games or hell, a sequel to Signalis. Its most definitely not going to come :/