Random Riddle: Big or Small

I can be real.
Or just a nice dream.
If you just use this thing.
Real – anything could seem.

I could be a monster.
Or an invention of times.
You can use me to think of ideas.
Or plot out huge crimes.

Without me, you’re dull.
Just don’t even try.
I just have one more thing left.

Do you know what am I?