I would love to see a contest to capture Classic Movie Moments.
This would be an advanced contest. Capture any memorable movie moment. A link should probably be added for those who have possibly not seen the movie.

My daughter helped me recreate this classic scene from POLTERGEIST.
Poltergeist was released June 4, 1982

The theme for this advanced contest is not mainly a physical object just light and shadows.

We live in an ever changing environment and things become timeworn quickly. I think it would be nice to focus our attention to those forgotten objects and use them as an advanced contest theme.

Here's one I haven't come across before: Fences. They're everywhere. Old, new, they come in so many varieties and can make interesting photographs.

Bonus contest? That's jus my preference as I like to do a bit of postprocessing on my shots.

This comp was a really good idea. Thanks Eury :0)

This would be a regular advanced contest in which entries would depict anything that has come to be known as beautiful or artful in a timeless way, thus making it a consensus classic. It could be a classic automobile as in the theme post, an artfully arranged stack of classic novels, someone dressed in white tie and tails, a classic building, etc.

Use HDR techniques to combine exposures of the same scene. Bonus Contest, naturally.

Interiors
There are a whole host of considerations when photographing interiors. On-camera flash units are no good, as they emphasise only small areas, and shadows abound from furnishings. The larger the interior, the harder it is to carry off.

I would say this would yield better results for an Advanced, or even a Pro or Bonus contest, allowing selective exposure changes. However, only a single image should be used...

Water is the subject of this bonus contest. No matter whether long or short exposure time, show us the effects that water can produce.

Insects are the most diverse group of animals on the Earth, with over a million described species. Show us your "one-in-a-million" insect shot.

Only insects (6 legs and a 3-section body) will be accepted, no spiders allowed!

(Regular advanced contest)

This is a Beginner level contest.
Machines are everywhere. We would be hard pressed to get through the day without using one. For this contest you are to take a photo of a machine or part of a machine. The machine can be old or new, working or derelict as in the theme post photo of an old ship's turbine blades.

Remember composition and lighting are very important. Keep the viewer's attention on your subject by taking care to avoid distracting elements in the background.