You can have a deep and complex campaign with simple parts. The way you make it complex is by complicating the interactions.

The simple parts in my campaign.

A misanthropic great druid: He hates people and wants them to stop destroying his forest

Barbarian warrior woman who usurped a small fief: Has her own code of honor regarding her vassals ( doesn’t really care about them so much as she cares about her own honor)  and she’s very competent

A wizard driven by vengeance: He hates the great druid but also wants to increase his magical power at the expense of the people who live nearby

 

Three fairly straight forward characters with straight forward motivations.

Now mix in the motivations of player characters. They become allied with the barbarian. One PC gets engaged to her. The party directly and indirectly thwart many of the plots of the wizard. Not because they dislike the wizard, his minions just happen to have desires at odds with the players. They perform tasks for the great druid because his desires mostly line up with the party’s and he doesn’t seem like too bad of a guy. He pays well and does favors for them.

Then the druid had the barbarian usurper killed by using the the party to get to her. The party decides they have to kill the druid now and, their best chance of doing that is getting help from the wizard, who does abominable stuff for his own nefarious purposes.

Powerful NPC’s with simple desires that most of us can understand or even empathize with.

Plus:

Other powerful NPC’s with simple desires that conflict with the other powerful NPC’s

Plus:

Player Characters

Equals:

Conflict which leads to complex interactions between the various simple parts. You don’t have to come up with elaborate plots or long back stories. All you have to know is the barest of details.  Character: What they want: Why they are having trouble getting it