D&D 5E How freely can a setting mess with core D&D mechanics?

[Yaarel]

I had planned a campaign like that once, but I never got to run it

There is much wisdom to having only one world. The one world includes disembodied spirits too.

 

[generic]

I would say that, as long as the group is okay with it, there is no wrong way. Heck, if you want a world where the casting of a 1st-level spell is nigh-world-ending, then roll with it!

In my opinion, settings should feel free to deviate rather heavily from the rules outlined in the base system.

 

[Connorsrpg]

Some egs at my table recently.
RULES
We use cards for initiative.
We have feat features 'kick' off some cards.
We use an Inspiration deck (with different things you can do on each card).
Use 100 XP per level.

SETTINGS
I recently changed halflings for one setting to be actual vessels for mephits. It is fun. Does not mean I do it for other settings.
Gnomes are not in this setting (unless they port in from Feywild).
I roll races for each of my settings from the Realm Charts I have put together and then kit bash from there, hoping each setting has something a bit unique, (even if that is not have a core race).

If interested in populating your world with races/sentient beings from many sources, check: Populations Tables and click on the links in the main table for sub tables of MANY races/species. (Inspired by the 2E World Builder's Guide - one of my favourite RPG books ever put together).

 

[Connorsrpg]

Very good topic, check out how I fought with my homebrew darksun conversion trying to be true to 2e flair so far. It covers things like weapon out of worse material, halfgiants, defiling and it really gave me headaches to find solutions which do not clash with 5e core mechanic.

Hey, nice, I too have been 'melding' 5E to Dark Sun. If interested in any of my conversions (I went with Size Large half-giants in the end), check out: Connors Settings - links to all Dark Sun stuff under Athas in the side nav bar. There are pages for races and even a 5E House Rules where I look at easy options for defiling, wild talents, non-metal weapon, etc.

 

[dave2008]

There is much wisdom to having only one world. The one world includes disembodied spirits too.

There is some wisdom in terms of game play, and I just like more tangible 'gods.' However, our reality is that there is not one world, so sometimes my since of fantasy realism gets in the way

 

[aco175]

For me, I find you go too far is when you loose your players. Official settings like Eberon, Thule, or Ravenloft have a core book that you can buy to expand the 'normal' rules, but a homebrew takes work to change more than some basic things.

I could have a cool idea for a underwater 'Aquaman' type setting with more D&D flavor. I first need to fluff the races to make them playable and them I need to change weapons and armor types to make them work. Monsters and transportation will need to change and them I can work on trade with other cities and farming rules. Eventually one of my players will say, "Which race is the same as a dwarf with a warhammer." I know that I have lost them and wasted a lot of time for something they do not want to play in anymore.

 

[Jfdlsjfd]

The Eberron setting book just came out, which has a few new character options, including dragonmarked subraces, and a variety of new gear.

But the setting doesn't change the core mechanics of D&D.

I'd be wary of large changes affecting the game balance between classes, unless playtested well. You can dictate how the world behaves, even if it's weird -- up to the infinite respawning at the Jorasco hospital -- but players get to choose their class and build their character idea. If the restrictions linked to a world make one of the characters basic concepts very underpowered and unable to contribute meaningfully, then I guess it's straying too much from the base rules.

For the rest, you can get wild as long as the world described is coherent with the rule modifications. You can't have the ubiquitous low-level magic of Eberron without some way to mass-produce this magic...

In the ZEITGEIST adventure path, one of the minor traits of the world is that gold rings block teleportation. If a person is wearing a gold ring, they can't teleport. If you surround a jail cell with a gold ring, someone can't teleport out of it.

That's pretty minor and explained in game during the campaign. It is good, as the world reacted to it and it was even shown to the player in the first scenario of the adventure path with the king removing his signet ring to be whisked away by teleportation. More serious change in the fundamental working of the world should bring more drastic reaction (a no-death world would be full of reckless persons and if you wanted to keep an enemy out, you'd develop a very secure prison system to make sure they can't just hit their head on the wall to "respawn" somewhere else...)