I'll preface my post by saying that I think it would be preferable to use more positive incubation methods, like daytime awareness/mindfulness and MILD techniques... but here is some advice for taking the nightmare route:

I induced nightmares a while ago so that I could confront some recurring nightmare characters (doctors.)
I used MILD incubation methods - mantras/autosuggestion, meditation, and giving my thoughts to the doctors daily.
I had a doctor as my phone's wallpaper so I would always be reminded to think about doctors appearing.
It didn't take long before they appeared in my dream, and I was able to confront them while lucid. (They never came back after that!)

I think you would have more chance of inducing nightmares if you chose something that's personally scary to you, rather than general horror themes.
It doesn't have to be something gory... lots of emotions scare people, like the fear of loss, death, change, uncertainty, etc. and I think emotions are more prominent in dreams than specific symbols or characters.
Stress is a common dream sign for me, and I pick up on it quite often and become lucid as a result, because I've recognised that my negative emotions in dreams are much stronger than in waking life.

If you're wanting to take a supplement route, nicotine is an option. Please only consider this if you are already a smoker, as it's a highly addictive substance.
People who wear nicotine patches to bed often report having vivid nightmares. I personally found that to be the case when I quit smoking using patches (I occasionally forgot to take them off at night.)

Like you said though, intentionally thinking about unpleasant topics all day would make life pretty miserable... So, refer to my preface for more appropriate advice.