Help, in dire straights

I'd like to say hello to this forum, and ask some advice.
When each of you tries writing a fiction story, where do you come up with the idea of what the bad guy is going to do for his "evil plot".
I understand that it'd probably be easier to write something in a world which I've created of my own, but, there are one or two characters in my own story, who need to get pushed out the door by someone. In this case, the someone is a character from an established fantasy world(Alustrial from Forgotten Realms), and working in said universe makes it easier and more difficult. Easier: I don't have to create a world on my own, Harder: It has to fit within the universe as it already exists.
My difficulty is coming up with a plausible idea/plot that doesn't completely change the chosen world.

welp first, start with your Hero's goal, then the details of your Hero and have the evil do the opposite. Like James Bond, defender of the crown, stop people from taking over the world. His villains try just that and to stop Bond, as far as what they do to accomplish that it can literally be anything and blow it out of proportion (depending on how big of a Hero your character is). So in Bond's case the villain has to have a big scheme, while an average cop may be involved in a small drug/money scheme.

If you're looking for something more outside of bigger bomb, top secret documents and looking for something like conspiracies then watch the news a lot... I get a lot from there. If you need a conspiracy, here's one that's still a little fresh... not necessarily total evil men, but men with good intentions but bad methods: Secret scientists think tank, acknowledges global issues with 6 billion people and only enough food for 4 billion and population growing exponentally they release a non-selective virus called AIDS for worldwide population control.

P.S. That idea is something I came up with so it's free to use by anyone that reads

Hmm, a virus created by man, that's meant to kill specific people/groups. I think I could make that work.
One hero (Alban), is a scholar at heart, but with a strong set of beliefs. And the strength to back up those beliefs.
Second hero (Soranic), a flippant and brash young man. The brashness is a cover to hide his insecurities.

Thank you for help love2code.

good luck and have fun.