Well, back to the (poor) parrots random proposal: one could make a motion graphics with the world itself, animate the head turn, do a lipsync, etc.
So could we have some creativity with rectangle layer. Rectangle deformations, an expressive rectangle-shaped character, a skeleton layer controlling a skeleton made of rectangles, an animation of Piet Mondrian art, etc. 
No, just a fool
The example themes? Because I didn’t proposed challenges. My initial intention once ago on my call for promotion of 1.4.0 launch was more specific: showcase of new features or even fixes of 1.4.0. No answers
That was exactly my point. Sadly, we failed. But, after @ekopimo suggestion, I thought we could make a call via Synfig YouTube channel. It has almost 20,000 subscribers. Maybe a new video post after so many months could drag attention of some of our few users that do not follow the forums.
Anyway, all of my original post was a proposal of proposals. If you, with your amazing skills on Synfig, does not like it, …
Regarding your theme suggestions:
Well, “Synfig logo” is, for now, the most voted theme.
These were in the challenge list Khemardi shared.
Good to test if the converter (still?) works as intended 
And to promote it.