Every rule below should be read as having the following clause at the end:
If you disagree - at all - come talk to me, and I'll see what I can do.That's no guarantee that I'll say yes, but I'm always willing to listen to ideas.
Every race or sub-race in an official WotC-published book is probably going to be fine. Yuan-Ti Pureblood is a bit overpowered, but if you have a compelling character, we'll make it work.
The campaign is ostensibly set in Greyhawk, but Greyhawk's high-magic enough that things like the Warforged/Shifter/etc. from Eberron, Loxodons/Simic Hybrids/etc. from Ravnica, they'll all fit in well enough.
All the official ones, including Artificer, are fine. Artificers are out of place in some 5e campaigns, simply because 5e can be lower-magic than previous editions, but Greyhawk's pretty high-magic, so they'll work out just fine.
The same goes for subclasses. All official WotC-published material works.
Since rolling stats tends to produce slightly higher numbers, but also doesn’t really hurt balance much (the game allows it anyway), I kinda wanna do rolled stats. But that has issues where some of you might roll really bad, and then Tyler get Tyler-luck-level results. So, my idea (not entirely original, I’ve seen mention of others doing this online) is this: All four of you roll standard stats at some point, 4d6-drop-lowest six times, and generate four arrays of ability scores. Then, at character creation, each of you gets to pick whichever one of those four arrays you want to use. None of the issues of rolling, should make slightly more heroic characters, and I’m more comfortable trying to balance things upwards to meet the party, rather than downwards because I mis-judged things.
Rolled stat arrays:
All WotC-published backgrounds work. Per PHB (Chapter 4, section "Backgrounds", subsection "Customizing a Background"), I'm completely fine with customizing the backgrounds' basic features. Pick an existing background, use it for things like starting equipment and the Feature, then just pick any two skills, and any two tools/languages.
Your character needs to roughly fit in with Greyhawk(ish) and have a reason to be a level 1 adventurer looking for work in Saltmarsh. If you want to tie your backstory more closely into the plot, just let me know what you have so far, and we'll work out a motivation.
You should have the usual two personality traits, a bond, a flaw, and an ideal. Pull 'em from the ones for the background you used, or make up your own; I'm fine with it either way.
I'm not going to outright dictate that your character needs to be Good, or not-Evil, or anything like that. But they do need to be someone who will work with the rest of the party, adventure for experience and profit, etc. So, no Evil in the vein of someone who kicks puppies for fun or anything, but if you're the sort of evil that will co-operate with the party in the name of larger plans down the road, or that will see the party as saps that're helping you gain power and prestige for yourself, etc., then Evil is fine.
This stuff's probably all fine. Run it past me first, but I'm probably going to say yes.
Less likely to just get blindly approved, but there's definitely some good stuff out there. Bring it to me, and we'll see how it goes.