This Ain't Your Daddy's BBEG

Anyone who has been a DM for more than 6 years or so probably learned that to make NPC's (especially ones meant for combat) you just pick the level you expected the party to meet them and create a character of that level. Before 5e, the challenge rating was directly tied to the character level of your NPC. It is no surprise then that many of us fall into the same trap in 5e, and teach others to do so. The thing is... NPC's are not player characters. I know that sounds obvious, but it means they do not have to follow the same rules for their creation.

NPC's Don't Need a Class

The classes that players choose from and level through are built to be balanced so that every player has relatively the same amount of power over the course of a "day". An NPC needs only to be balanced for a single encounter, and this balance comes from their CR. This means if you want to give it a power that is for a "higher level" you just need to adjust it to fit their CR. They do not need to have levels so if something fits the concept you have give it to them. This also means they do not have to multiclass. You can create interesting character concepts by combining aspects of multiple classes without worrying about how those abilities "balance" compared to anything else.

You Don't Need To Plan For Everything

The only thing absolutely necessary when first creating your NPC is a brief backstory that nails down the character concept, and a stat block that has all their combat abilities. You don't want to make up combat abilities on the fly. Your players trust you to be fair, especially during combat, and that trust is hard to get back once lost. But you probably don't need to know exactly how your oracle sees the future... only that they do. You can decide what out of combat abilities best suits your story when the need arises. There certainly is nothing wrong with choosing every detail before the game starts, but if done right doing it on the fly makes it much easier to go with the flow of the story.

You Especially Don't Need To Plan Gear

This refers mostly to magical items. Unless a particular item is a signature piece of gear for your NPC, it does not need to be taken into account in their stat block. Assuming you started with the CR, all their numbers are based on it. This frees you up to just give that NPC the items you want to use as rewards without having to worry about if they are too powerful against your players. Want to give your player a +3 sword? Give it to them after defeating the baddy. They will just assume the +3 was taken into account in your math. This is especially true for character specific items. If your hobgoblin warlord came across a mace whose magical properties can only be used when attuned to a Paladin of Bahumut, he will still wield it. It would still be more powerful than any mundane weapon his tribe had, and now your Paladin player has a special weapon he got, and a good story for where he got it. No stat block adjustments necessary.

Do Not Abuse Your Power

With all this said about the freedom you have to create what you want, this is probably the most important piece of advice. If every NPC you make is a god that can do anything that they want whenever they want, eventually your players will stop having fun. Every NPC should be unique, and should be limited in power by their backstory. Use abilities from different sources to create an interesting character, but if your BBEG can wield every weapon, raise the dead, see the future, AND perfectly track anything on any plane of existence, you have probably gone too far. When deciding what an NPC can do, ask yourself does this fit this character's concept, and does this serve the story? If the answer to both is always yes, you will probably be fine.