Most NPCs in a campaign tend to be some kind of humanoid. Whether human, elf, drwarf, or any other humanoid, they are used to help flesh out the world. The problem though is that they are incredibly versatile. A human can be anything. In contrast wights, vampires and liches have their own applications that make them special. These imply and inspire stories due to their uniqueness. Humanoids are more difficult in this way precisely because they can be anything. And it is for that reason that I hope to explore this topic. I also know they aren't very monstrous, but hey, we Dungeon Masters throw them at our players so why not?
Look Somewhere Else
The first way to address this situation is to not rely on the humanoid background for your story. Instead, it needs to come about through other means. If you run enough liches, you'll run into a situation where you don't know what else you can do with a lich. However, that doesn't mean you will never run a lich again. Instead the focus turns to the character of the individual lich in question. What are they after? Who do they hate? Why are they doing what they are doing? Here the difference is between what you can do with just any lich, and what a particular lich might do. Simply put, you need to come from the character first. I find myself in this situation very quickly when needing to make humanoids.
Elves, drwarves, humans and other humanoids tend to have something that makes them distinct from each other. Further, even in the core rules, there are further divisions in terms of different types of dwarves and elves. This is immensely helpful for me. Instead of thinking of a human, think about what a resident of Baldur's Gate would do or say. If the type you've chosen is too general, go more specific.
Let's Change Things Up
If the nature isn't inspiring, you can change the nature. Perhaps your humanoid baddies are the reject descendants of gods trying to prevent your players from achieving divinity. The key here is to go more specific. Human might be very general, but godkind may not be. In the core books there are many kinds of humans and likewise you can do the same. It's not just a war between humans on one side and humans on the other. It's two distinct cultures with their own values. As a result, they are no longer just humans. What we are doing is making the type of humanoid new by changing them. Of course, the extent we'll need to do this to help inspire ourselves will vary depending on the Dungeon Master in question. This is a bit different from the above option but very similar. There, we went more specific to find something to inspire us. Here, we make up something new. It will be more specific, but it doesn't exist yet. Often this takes the form of a subversion. Good orcs, good succubi, etc.
One humanoid baddie by themselves often isn't enough to challenge a party. Instead you throw 4, 10, or 40 at them. And when you have that many humanoids, they'll have a social structure of some kind. These interactions and structures are some of what makes humanoids so fun. They aren't undead skeletons who are probably the world's worst standup comedians. You also don't want them to be the same as undead skeletons. Naturally personal motivations also come along with social stuctures. Thinking about both social structures and character motivations is a path to many an interesting adventure. Even a necromancer might have apprentices with their own relationships with each others and others outside the organization.