[MENTION=5142]Aldarc[/MENTION] exactly that.
Eberron has colored Dragons, but they are not necessary of eveil alignment. They got Orc druids. Hobgoblins, Kobolds can peacefully roam around in cities. Drow revere scorpions instead of Spiders. So many things are there but most with a Twist.
Fluffwise it may be the Setting which is hardest to converse, e.g. you just cannot pick the Monster manual and select a red Dragon as the next oponent because chances are that he is one of the good guys.
So that is totally against the core rules by [MENTION=7635]Remathilis[/MENTION] Definition.
Except its not.
Remember, a world COLORS the core rules. Dragons not necessarily being Evil colors the perception, but it doesn't change either the mechanics or the assumptions of dragons. You're not saying they are 10,000 feet tall, covered in pink fur, breathe hot buttered rum as a breath weapon and can only talk sdrawkcab, they are still dragons with a potentially different outlook. Drow might patriarchal scorpion-worshipping jungle-dwellers, but they are still majority EVIL and fill the same general antagonistic niche the MM ones do.
I get most people want to represent my point as "The PHB is inflexible and ALL games must obey it", but its a lot more nuanced than that. To whit.
Player: Hi, I'm new to this group but I have my PHB. What are we playing?
DM: This game is set in Eberron.
Player: Oh, Ok. I wasn't expecting that. I made a halfling rogue...
DM: That's OK! You can play a halfling, they tend to be nomadic dinosaur riders, but some live in cities like everyone else, or are scouts...
Player: Wait, did you say DINOSAUR RIDER?!?
DM: Yeah, they use small ones as mounts.
Player: That's cool! I want a dinosaur mount? Can I get one?
Sure, you'd need to talk to House Vadalis to buy a mount...
Player: House Vadalis?
DM: Yeah, houses control various trades in Eberron, each house is made up of an extended family, usually by race...
Player: Are their halflings ones?
DM: Sure. House Jorasco are healers, and House Ghallandra are hospitalers.
Player: Cool, I want to be from House Ghallandra and maybe I'm a rogue because I stole money from guests or something...
DM: Sure, we'll hammer those details out.
Compare to...
Player: Hi, I'm new to this group but I have my PHB. What are we playing?
DM: This game is set in Dark Sun.
Player: Oh, Ok. I wasn't expecting that. I made a tiefling monk...
DM: Sorry, but tieflings don't exist in Dark Sun as its removed from the planes. No plane-touched races do. And Monks aren't part of Dark Sun's feel and they aren't hampered by primitive equipment. Sorry, you'll need a new PC.
Player: Oh, uh. Can I be a half-orc or a dragonborn? How about something like a bard or warlock?
DM: Sorry, neither of those races exist, and bards are really rogues with the assassin subclass and warlocks are all Templars. You can be a bug-person or a half-dwarf!
Player: No, that's fine, I'll play a human fighter.
DM: OK. Keep in mind you can only be a champion or a battlemaster, as there are no samurai, cavaliers, arcane archers, or eldritch knights on Athas. You can take Psychic Warrior though!
Player: Actually, I think the local game store is hosting Friday Night Magic. I just got a new merfolk deck I want to try. Thanks anyway...
Both worlds created characters that would fit their setting; but one did it by offering interesting additions and changes, the other just kept restricting the player's options to ones that "fit". One feels welcoming, the other gatekeeping.