[MENTION=7635]Remathilis[/MENTION] #474 "Except it's not...."
The way i do it is different. See, i discuss the General outline of a Setting and how i intend to run it upfront. Then i create a Matrix with playable race class Combos.
This i give to the Players and ask them to find something nice. They also discuss with each other to get some sort of Balance to the Group. If anyone is missing anything i will check if i can somehow shoehorn it in, but that has not been the case yet.
I do include things if they make sense, e.g. i dm Greyhawk atm allowed Player races are human halfelf halforc tiefling Gnome and that's it. All of them are accepted in the free lands. I use the official material as a Frame for a classic good vs evil campaign in a qasi Renaissance Environment. Iuz and his orc hordes bolstered by demons have plundered and marauded the lands.
Each of the Special races makes sense: Halfelves and halforcs are mostly orphans of war. Dwarves and elves exist but only as npcs they few and often remote.
Tieflings got to have lawful Background (No one wnated to Play one yet but if) because otherwise they would be associated with the enemy.
It all fits into the Settings published material the feel is a bit different but you can recognize it is greyhawk and you can recognize it is not FR, and most improtant my Players and me enjoy it.
But let us get to your example:
Assume:
Player: i got this Halforc Barbarian (Not an unusual choice)
DM: To bad, we Play Dragonlance, sorry mate no Orcs on Krynn you can Play a Minotaur instead.
- So you see that is not DS specific
Another:
Player: I want to Play a tiefling Monk
DM: hm we Play darksun. So you want some martial Monk with unarmed combat? How about a Thrikreen Gladiator?
Ah, you want an elemental Monk? How about a Druid or an elemental cleric? How about some sort of mystic?
- So this is also resolvable in a contructive way, if you got mature Players.
You can tweak some of the classes allowed with a custom Background also if something is amiss, 5E is a Toolbox and the best Toolbox of all Editions so far to do such things, except eventually 4e where all things just had different names but i am not to familar with 4e.