How does your submission match the theme?
My suggestion fits the theme of dual purpose design, because the jump button is linked to several actions. Of course you can jump, but jumping moves the floor limit downwards. So jumping is literally needed to progress the game. No only that, but you can only fire while in the air using the jump button as well, just like Downwell. Shooting downwards while in the air gives you a little jump boost.

Enemies drop ammo when you kill them, so shooting is encouraged. The ammo-drops also kill enemies below you, so you don't have to spam the jump button when there are multiple enemies below you. It's extremely satisfying to see a whole pack of enemies disappear after shooting one of them

Another twist is that you have to shoot heart containers to retrieve a bit of health, so jumping is required for that as well.

In conclusion, you can have a lot of actions performed with just one button. In this case, the jump button.

Third-party resources
During the making of PJTF I used:
- GameMaker: Studio 1.4 for development (https://www.yoyogames.com/);
- Paint.NET for graphics (https://www.getpaint.net/);
- FL Studio for music (https://www.image-line.com/);
- BFXR for sound effects (http://www.bfxr.net/).

Within FL Studio I only used the NES VST plugin by Matt Montag (https://www.mattmontag.com/projects-page/nintendo-vst).

Contributors
GameMaker: Studio by YoYoGames Ltd (https://www.yoyogames.com/)

Paint.NET by dotPDN LLC (https://www.getpaint.net/)

FL Studio by Image-Line Software (https://www.image-line.com/)

NES VST plugin by Matt Montag (https://www.mattmontag.com/projects-page/nintendo-vst)

BFXR by increpare (http://www.bfxr.net/)

Game development by Thomas 'TSJGames' Sjerps (https://tsjgames.itch.io/)

Source Code
https://drive.google.com/file/d/0B18DvAAeDaH7UHpvdWpNNEF5RE0/view