It doesn't even have to be "harder" or "more complex" it could be as simple as "Take x way into the basement for a mini boss or pass it via the stairs and fight trash instead".
Yes, that is directly from WoW's Shadowfang Keep, and you know what? My groups took both ways, to this day (♥ WoW).

I ike this game... but aside from looking pretty, the dungeons have never interested me. Boring, slogging themepark.
Dare I say even Tera has better dungeon design.