I'd say bring the trolls.

I did some asking around and found out the average nature of trolls (I've not played much of the games, nor have I read any of the books, but I plan to). I'm gonna make them a monster race, but someone could bring in a troll that, unlike others, does travel. But since they don't really travel, I'm gonna make them a monster race. Keep in mind that monster race doesn't mean you can't make a character that's a troll. You can play any sentient monster in the game, but you'll have different reactions based on what sort of monster you're playing.

I have found a fifth race that can be a player race, though. Sorcerers would be a separate player race, since not every human can be a magic user. Since magic changes the person, it would be considered a separate race, just as Witchers are considered separate.

Not really fond of DnD, but very fond of the Witcher (books and games). Dryads are a potential player race too, I'd say, as are Gnomes (distinct from Dwarves). Balancing Witchers and the other races/classes seems like it might be a pain, though there's a fair amount of competent non-modified humans in the setting.

Okay, why is it you're not fond of D&D when it has a ton of freedom involved? Are you going by 4e or something? Because that's not worthy of being called D&D with all the restrictions on creativity it has. I'm using 1e because it presents a lot more freedom for rule changes and can easily integrate custom classes as well as adapt other settings to it (there are actually some people working on a modern adaptation using 3e at the very latest. 3e is when the creative potential began to go downhill).

I'm thinking about having regular, non-magical humans having the same advantage that they have in regular 1st edition D&D, meaning no level cap. Witchers, Elves, Dwarves, and Sorcerers will, however, have a level cap.

Witchers and Sorcerers will also be limited to a single class. Witcher and Sorcerer are to be both a race and a class. Only a Witcher can be a Witcher, but that's also all they can be. Of course, they could put focus on Signs or Combat, but they'll still have to mix the two. Sorcerers can wield simple weaponry, but their greatest assets are their magic, which is to be the same as default D&D spells since the Witcher's magic is similar to the stuff used in D&D (with the exception of the Signs, which are a special case).

As for balancing, the games did a fair job of balancing Witchers, but Sorcerers are gonna take some work to balance out. Fortunately, 1e is very forgiving when it comes to powerful characters.

I'm thinking of making the non-human level cap around 30 or so. It should, however, be noted that it could take months of owning a character to get him to level 30.