First off, it’s cool you’re doing this! I wish you the gift of the Muses!
Anyways, I personally like the sweet spot between child friendly and adult. Perhaps like PG 13. It could reference more mature matter but not necessarily have to go all the way with it.
Of course if you want to delve into darker subjects, an M rating might work.
If you want to reach out to a broad audience you could try a PG or even a PG-13 rating if you want kids to be able to watch it. Personally I like PG 13 since I like the balance but it might better to have an M rating for more freedom. You can always change the rating in the next season.
As for nostalgic fantasy, I like Victorian era or 1700s style fantasy. Honestly I like a lot. But you could give Eastern European or Indian or Indonesian vibes.
Perhaps there could be some exploration. I have been looking at Point and Click adventure games like Syberia and Longest Journey and how they have beautiful atmospheric worlds.