Hey it's my firs time commenting in this part of the forum (all because of that beautiful title) now let's get to it. Dark Fantasy is good, Dark Fantasy playing as an "evil" creature is even better! I follow some comics/manga with a similar concept (most of them already mentioned here) so i'll probably enjoy this too. To answer your questions:
The "starting point": if you want the characters to interact more with "monsters" (with humans/elves being introduced mostly as enemies) then the "monster lair" setting is good but if you want them to interact (more or less peacefully) with humans&co then an "adventuring party" setting would be more "fitting" (and easier to pull off) imo.
World: it should be unknow to the players (except some areas near the "start") who didn't travel/explore (this can be one of the innate traits) the ones who did should know at least a little more (for example: an ancient Vampire/Lich would certainly know more than a goblin)
Approach to the game: i would much prefer a linear one as some players can ruin the game for everyone with their choises.
Classes: i know nothing about the Gaterras system but if it works why change it? Of course the names (and maybe some of the abilities) should be changed (to make them more "evil")
Strenght of the characters: OP protagonists/players become boring very fast in my experience (well written ones are an exception of course) so i think they should have some weaknesses that the "heroes" can exploit.
Races: you are going for a "monster game" so all the mosters capable of intelligent thought should be included i think and the ones that are not could still be included under special circumstances (like a blessing/curse from an evil God)
Magic: again i know nothing of that magic system but if it works in this "new" enviroment you should keep it (while changing some of the spell names of course) it would be easier to make/organize if you work with something familiar.
Project name: seeing as the current name is what brough me here it's probably ok to leave it as it is (well Dark Souls would be cooler but it's already taken), there is one consideration though, if you want your players to "be" evil the title is perfect but if it's about them "adapting" to the humans (or trying to survive in general) then something more "neutral" would be more appropriate...like idk "misfits" maybe?