Ability Scores: Unless I say otherwise for a specific campaign/adventure, ability score generation is Point Buy.
Alignment: D&D5e generally doesn't care about alignment much, so I really don't care whether you actually choose one. There are more than nine personalities, and players too-often end up deciding on actions because of their alignment, leading to a player only having nine personalities that they roleplay, which is silly. Ultimately, I feel that the Traits/Ideals/Bonds/Flaws thing does a better job of fleshing out most characters. That all said, if you wanna set an alignment on your character sheet, go for it.
Customizing Things: If you wanna do something like play an Int-based Warlock, Wis-based Paladin, etc., or you wanna use a class for something other than its default flavour, because it has the best mechanics to represent what you want to run, then feel free to run it past me. I'm open to a decent amount of bashing the system into working.
Classes and Subclasses: Unless there's something specific to the adventure, that makes some option not work, all classes/subclasses officially printed by WotC are allowed. Similarly, since it has D&D Beyond support, the latest Blood Hunter class is also absolutely allowed.
Races: Everything from the PHB, VGtM, EEPC MotM, or other non-setting-specific stuff is valid, unless I said otherwise for the specific campaign/adventure. Setting-specific stuff is almost always going to be allowed, even in the wrong setting (with adaptations as needed), but it's worth checking with me first.
Tasha's Cauldron Optional Rules: In general, all the optional/additional class features from Tasha's Cauldron of Everything, are fine.
Spells: Similar to races, anything from the general, non-setting-specific books, is allowed by default. The big exception is, of course, Dream of the Blue Veil. As for anything from setting-specific, again it's almost always allowed, but you should check with me first. Oh, and if you want to use Silvery Barbs from Strixhaven, it's a second-level spell. All the others in that book are, and it's just too good as a first-level spell.
Oh, and in general, the various Conjure Whatever spells from the PHB are just bad design in a bunch of ways (monster stats aren't balanced by CR the same way as player features, cracking open the MM mid-combat sucks, adding eight more things to intiative slogs the fight down, etc.), so generally players should go with the Tasha's Cauldron-style Summon Whatever spells.
Background: The custom background stuff from the PHB is always allowed, as is basically any background from any source that works for the character you're envisioning. If the background is setting-specific, but almost exactly what you want, talk to me and we'll work out a custom version that works just as well.
For anything WotC have declared Legacy Content (I.E. races and other things where they've published a more-recent version that they feel works better), I'm probably okay with either version.
In general, note that if you're using any of this stuff, it may or may not work on D&D Beyond, which I generally use. Their Homebrew features allow a lot of customization, but have limits. This is fine. You can use a good old-fashioned paper character sheet, with a proxy character in DDB that has the right stats/spells/equipment/skills/etc. for me to track stuff, but without working features, and that's enough for most purposes.
Unearthed Arcana: There are a bunch of subclasses, feats, etc. that were published as Unearthed Arcana (I.E. Playtest) by WotC, but never got released for whatever reason. In general, any character option that wasn't published, I'm probably open to using. The link above is to a master list of every UA published before WotC started the current OneD&D stuff, so you can look there to see what there is. The article links mostly don't work, since WotC breaks their website regularly, but the direct PDF links do, and I have archived copies of all of it anyway. So yeah, if you want a martial-esque Stone Sorcerer, let me know; you can probably make one.
Homebrew: No using anything without pre-approval. Anything widely community-accepted I'm probably okay with, and will likely approve, depending on whether it fits the campaign/adventure. If it's paid content on the DM's Guild, I may well not own it, and purchasing a copy is your responsibility. A list of what I have already purchased, is below.
If you wanna use any of this, ask me about it! This is just an exhaustive list of stuff from the DM's Guild that I own and think is worth mentioning here; I haven't necessarily read all of it closely enough to know if I'd 100% approve it for use.
Eberron Stuff:
Other Stuff:
Multiclassing and Feats: Allowed. Some setting-specific feats may need approval, as with other setting-specific stuff.
DMG Combat Action Options: Climbing onto other creatures and Shoving aside are 100% allowed. Mark isn't. Disarm, Overrun, and Tumble may be allowed with prior discussion; I haven't used them yet, so am not entirely sure, but willing to at least try them out once.
Xanathar's Guide Tool Uses: XGtE lists a bunch of options for how to use tools (Alchemical Crafting, Potable Water, etc.), and those are pretty much all
Skills: In general, 5e doesn't really require that certain skills be used with certain ability scores. However, things like the official character sheets, the D&D Beyond stuff, etc., often assume that any Medicine check is an Int(Medicine) check, e.g. I don't really follow that. If you want to perform surgery, that's a Dex(Medicine) check. If you want to recall information about a song, that might well be an Int(Performance) check.
Other Proficiencies: Similarly, proficiency in tools, or even weapons or armour, may apply to some checks. If you're trying to recognize the crafting styles used in a strange suit of armour, you might make Int(History), or else Int(Smith's Tools). Obviously, for any check that's more than just knowledge or understanding, tool proficiency probably can't apply without you actually having those tools.
Extra Proficiencies: As per the first recommendation in Xanathar's Guide, if you have both a relevant skill, and a relevant tool proficiency (including having and using the tools as appropriate), will give advantage on checks. You're not limited to the skill uses listed there, though; any time both are relevant and would provide your proficiency bonus, you can get advantage.
General Procedure: As a result of all the above, when you tell me something like "I do X", I'll oftentimes just ask for an ability check, not mentioning any skill. This is because remembering every single proficiency off-hand is just not something I can manage, and I'm not gonna slow down every check by trying to read the list and work out what applies. To that end, if I ask for a check, and you think you have some proficiency that applies, feel free to ask if it does.
Oh, and in general, if you just ask something like "Can I make an athletics check?" or "Can I Insight him?", the answer will be that yes, you can make any check at any time. I can't stop you from rolling dice, after all. But if you want it to mean anything, you'll need to instead describe what the hell your character is doing.
Help, Guidance, etc.: In order to help someone with any kind of check, you're going to need to be proficient in the thing they're doing. You'll also need to spend as much time helping, as they spend doing. As for Guidance, the biggest thing is that it's very obviously a spell. If you try to cast it mid-conversation, the other person is going to be angry. Plus, of course, it's something you do need to cast in advance.
Multiple Characters Attempting Something: In general, you can't. Specifically, two characters can attempt a task, and one of them will count as hing the other; see above. If someone tries to convince the NPC, pick the lock, smash the door, etc., and fails, you can't just have everyone else roll hoping to get a nat 20 and have someone succeed. Instead, you'll need to change the circumstances somehow, in order to have a second chance.
Inspiration: D&D5e only allows for Inspiration to be used by a player on their own rolls, and always ahead of time to gain advantage. In general, I'll allow anyone's Inspiration to be used on any PC's roll, and I'm okay with using it after the roll's already been made (assuming it wasn't made with advantage/disadvantage) to add the extra d20.
Recharge: The default handling of enemies with Recharge abilities in 5e is silly. The DM is supposed to roll the recharge die at the start of each of the creature's turns. That means there's no way for the party to take advantage of the recharge, since the enemy might have it up on any of its turns, without warning. To that end, my method is instead to roll the recharge die ahead of time, after the end of each of the enemy's turns, rather than wait for the next. If the recharge happens, I'll describe something to telegraph that the big powerful attack is ready ("The dragon takes in a biiiiig breath.", "The sounds of whirring from inside the clockwork golem speed back up.", etc.).
Being "In-Combat" / Initiative: In general, rolling initiative doesn't necessarily mean we're in combat. It just means the party is in a tense situation where we, the players, need to keep careful track of what happens, in what order, moment-by-moment. In particular, if the party runs into a complex trap, initiative will probably be getting rolled.
As a result of that, my position is that anything you can do in initiative-tracked time, you can do out of it. And anything you can do without rolling initiative, there's a way to do once we're in turn order.
Spell targeting: Some spells specifically only target creatures, but others allow both creatures and objects. This is sensible for some spells (Hypnotic Pattern can't affect a chair), but a lot of single-target damage spells don't make much sense as not being usable against objects. The problem then gets worse for things like mimics - RAW you can just attempt to Eldritch Blast every object in the room, and only the object that's a mimic will be affected. So, my policy is that most targeted damage spells, you can use against objects. Unless a spell does psychic damage or something, assume you can.