THINGS TO CONSIDER
That's a great question, thanks for reading it.
Almost everything is homebrewed.
The world, magical items, the monsters (in every sense of the word), the lore, and most importantly the characters, the story, and the mystery. All from scratch.
However, races, classes, skills, spells, basic equipment and weapons are still what is in the PHB.
Assume no prior knowledge.
Remember when I said Feldereach was a continent, and it is insulated from the rest of the world? Well to be as blunt as possible, I didn't build the world further than Feldereach. That would be too little jam spread over too much toast, and that is unfair to you as players. Everything takes place on this continent, and the game and story ironically will be richer, focused and alive because of it. Rest assured though, there are all kinds of geography in the land, with hopefully something you've never seen before lurking in every crevice. What do I mean by assume no prior knowledge in this context then? Well the fact that in our story there is only Feldereach means there are no Forgotten Realms, no Faerun, because we're not in those places.
This is a curated world, and story. (Again, assume no prior knowledge.) If I've done my job, everything will seem like it belongs in Feldereach. Therefore, there are things that are in D&D that are not in Feldereach, and this should help you in deciding what would be a good idea to choose for a class, a race, skills, spells, etc.:
Some examples: there are no planes, dead is dead, and the gods listed in the PHB don't really exist/are not believed in in Feldereach. If you'd like to make a character who subscribes to a religious belief or a god, feel free to make one up. Just know the typical D&D gods you've come to know do not play a part in our story. This is a story about people, monsters, and the intersection of the two. If you feel you can't make, for example, a cleric work without a god, we can discuss.
This is getting too wordy. Just tell us what the world is actually like. What types of people and monsters live in it? Would the race I'm thinking of really belong in Feldereach?
You know what? I'm going to word you even harder. This is a dark, twisted fantasy world. Without spoiling anything, think of the general vibe of Lovecraft, Universal, Hammer, and Price horror. Germanic and Grimms' fairy tales. Remember the dementors and/or the thestrals from Harold Potter? Now we're cookin'.
I don't know what those high-falutin' things are, give me D&D examples of what would and would not belong in Feldereach.
You would not see a Yuan-Ti, Kobold, or a Beholder. You would see a vampire, werewolf, or a wraith.
BUT LET ME BE CLEAR: while I highly-encourage you to let all these strange examples soak into your stupid blood and to consider them when making your character, you can make whatever you want. If it's really crazy and I don't think it'll ever fit, we can work together to find the sweet spot. I want you to have fun!