I had a lot of fun getting to directly toy with video game physics in this miniature experiment of yours. There's a lot of potential here to expand on what this demo already offers, so I have some ideas of my own to offer.
1. Give more advanced movement mechanics to work with, such as air dashes, ledge grabs, ground slides, jet boots, and more.
2. More movement presets from games that you covered before, as well as some suggestions of my own like the Fancy Pants Adventure series, Phoenotopia: Awakening, AdventNEON, and UNITRES Dreams.
3. A level editor that allows sloped and curved geometry in addition to regular flat surfaces for more level design possibilities.
4. Physics based objects like crates and ropes that the character can interact with.
5. Variable movement speed with the analog stick to allow for subtle adjustments when doing tight platforming challenges.
I'm sure that more concepts can be offered, but this is sufficient enough for me. I look forward to this neat gadget being the Mario Maker of indie games. Perhaps you can even upload it to Newgrounds if the site interests you.