For me, the story and characters are the fun bit, and I stick to the mechanics as they already exist for the most part. But the names... I struggle naming one character. Naming large numbers of characters in a way that feels cohesive is hell.

I actually had a way harder time adapting a story vs coming up with my own. I took a premade adventure that didn't really have a "story", just a bunch of quests the PCs could do to level up before fighting the big bad, and tried to make an actual arc out of it. It was really rough, and some of the quests you could tell were reeeally stretching it.

Came up with an idea for a bad guy. Looked up some lore to see where this guy may have come from, what type of being he may be, etc, and from there came up with a pretty cool back story, a couple of assistants/ underlings, and one or two plot hooks. It's nowhere near done but it's already way more interesting with more room to explore than the "easy peasy rearrange stuff" module I attempted.

Pull a page out of Disney’s book: dead parents.

In what way could this character have lost his parents in this universe? Vary the emotional damage to fit the theme you want (you can even have the character’s parents die in a stupid way if you want the story to be somewhat humorous)

Create a villain (or hero) to have committed that act—or use factions you already thought of—and voilà you have a character revenge arc set up.

You can repeat this with basically any universe, and it still feels fresh if you just keep changing a few things around.

Did it happen recently to this person or a long time ago? Maybe change it to be a different family member or a friend. Maybe this was a massacre where there are multiple survivors seeking revenge. Maybe instead of seeking revenge they are really just trying to hide because whatever villain took their loved ones is hunting for them still.

Hell you can even push the entire thing into the past, turn the revenge arc into the lore of the world if the completion of that revenge arc would have had a large impact. Then the main quest could be trying to undo what has been done, or trying to stop another group from trying to undo what has been done. You could even obscure the actual events and make legends that all are loosely based around the truth but contradict each other so your characters need to uncover what really happened. (Cultures like having myths that make them seem better than they actually were)

When you need random characters to play roles in the plot, its often fun to just pick a random race/culture/class in your world and then say “how could a person from this group have ended up where I need them to be and with the skills I need them to have?”

Random name generators ftw

Check your local library (if you're lucky enough to have one). Mine didn't have any rpg adventure books in stock but they let me search their online database and I found some that looked interesting. They shipped them in for me to borrow, at no cost to me. Amazing service!

It's a great way to try different adventure books before buying the ones that suit your style. Some adventures are quite fleshed out and some are bare bones with plot holes you can fill however you want.

Another thing that can help is to bounce ideas with other DMs or players that aren't in the campaign: give one a call if you know them irl, or make a post online, if you get stuck and need inspiration. Two heads together are greater than five heads alone, believe me.

Finally, take notes. Inspiration can come from the oddes sources, so write down anything and everything that might be usable at some point in the future: talk during the campaign, overhearing gossip about a stranger, books, movies, poems, games, dreams, working on a problem at work/school. Reading the notes back later can either be gibberish or lead to an idea to use.

I know I love a character if I end up building a world around them. Which in the case of my cat, I'm absolutely doing.