And trying to explain for the seventh thousandth time, why comparisons to other games are being made, is that a lot of use do not see D&D as setting or a genre, but a rule system.
A rule system with a purpose.
IF you said you wanted to run Call of Cthulu... I expect insanity rules. I expect cosmic horror. I expect magic to be horrible and dangerous.
If you said you wanted to run DnD... I'd expect a adventure with 4 to 6 people, each playing different classes, with a wide variety of races, and reliable consistent magic.
The game system wasn't designed to be generic. This isn't an engine like D20, Savage Worlds, or Fate. DnD is much more specific than that.
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Because of how much I like Dragons. They are very rare, powerful and majestic creatures. They are the iconic creature in D&D and one of, if not my favorite creature in the game. To see their blood reduced in majesty and power that badly and made relatively common in the form of Dragonborn, greatly reduces that for me.
Which is why my most recent idea of Dragonborn being created servitors of Dragons, whose majesty and mere prescence warped people into taking on Draconic features, is something I think is kind of cool.
So, you want to stop people from playing dragons becauses dragons are so awesome, but other people want to play dragonmen because dragons are so awesome. Your reason to say no is the reason they are asking you to say yes.
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The game was designed with a goal: to simulate picaresque pulp-fantasy adventures wherein each player controls one miniature figure, representing an adventurous treasure-hunter exploring the dungeons underneath the proverbial "huge ruined pile, a vast castle built by generations of mad wizards and insane geniuses," until said character either dies ignobly or can amass enough wealth to carve a dominion out of the wilderness, finance and field a private army, and go to war with the other high-level player characters' private armies. Certain later editions do rather a poor job of it, but it's what D&D was invented for.
The idea that D&D can't have a good crafting system does give me a bit of a chuckle, though. Can't help it. My preferred milieu when I play is Victorian steampunk, and you can't properly do steampunk D&D without a technologist class designed around building gadgets and inventions. So… yeah, as with your "gods & galaxies" example, been there, done that. Keep throwing ideas at the wall, though, and maybe you'll hit upon something that D&D really can't do. (You might have a rough go of it, though, given the fact that both the 3rd edition d20 System and the OSR have already gone hog-wild adapting one or another of D&D's rules engines to every genre under the sun.)
It is almost amazing how you can read a post and get it completely wrong.
Me: "I know there are people who believe in set style for DnD, because I want a crafting system and have tried to implement one, but I find it hard, and the advice I have gotten on this forum is 'Don't do it because DnD isn't a game with crafting systems'"
You: "Well, I play a steampunk game that uses a crafting system, so you are wrong that DnD can't do crafting"
Well, if I had ever said that, you would have made a cutting point. But since that wasn't the issue, wasn't the point, and addresses nothing... then it was kind of a waste.
So, to reiterate, there is a style of game that is DnD, people on this forum believe that too, and my personal experience shows one of the things that it seems like the majority of people on this forum believe is that DnD isn't for crafting systems. So, saying "DnD is a system and that system can make any possible game in existence" is just not how most people see this game, or how this game was designed.
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As I said something might exist you don't get to play it.
Theros for example the only has 6 races iirc, it does say anyone else is a world traveller. That doesn't mean you get to play such a traveller.
The theme is Greek based so something like an elf doesn't fit.
Geo locking something just means the focus is on this area and races.
Race might be banned because it doesn't fit, DM doesn't like it, mechanics, DM doesn't like it's appearance, DM doesn't like the font used, doesn't like the smell of it etc.
Elves don't? What if they were the child of a Dryad and a mortal hero? Or the God of Nature and a Princess? What if they are Anvilwrought and crafted by Purphoros? Carved from marble and brought to life by a god to reward a pious mortal.
I mean, that is four different ways that an elf could perfectly fit into a greek based story. I mean, it isn't like "pointed ears" is a hard concept when we look at Dryads, nymphs, and satyrs.
And in order of your reasons 1) debatable as we show 2) Very rare I think 3) Very very poor reason 4) and 5) stupid reasons. Especially since DnD uses the same font and smells for every book.