Generally, the official 2024 D&D 5E rules are followed to the letter (or number) as closely as possible, unless there is really good reason not to. The "Rule of Cool" absolutely fully applies to story, character, flavor, and everything outside the rules—but to me, to "Calvin-Ball" it and change the rules on the fly is to lessen the stakes, challenge, and even authenticity of the experience as a game that involves rolling dice and all that.
That said, "House Rules" are something different—these are established and agreed-upon from the outset.
Ask me before choosing a changeling or flying species. They might not be right for the game in question.
I'll review every character and suggest or make adjustments to fit the setting best, maintain some degree of balance/challenge, or any other reason.
Generally, if you can build it on D&D Beyond (I have and can share any of the content on there), it should be good, assuming you aren't trying to downright exploit it. No laser-rifles, for example (nice try!).
I'll assign magic items, gold, etc. after you've built and "turned in" your character.
Feel free to ask if you're ever unsure. I'd rather you check first than fall in love with something that we might need to change later!
I'm fair and tasteful but this is a fantasy game and will definitely involve mature themes like death, disease, subjugation, horrific creatures and events, and the like. Consider it rated mostly "PG-13" with a fair dose of "R."