- Biggest feature that I'd like to see is a leaderboard. While it wouldn't be fair to allow me to get on the leaderboard after already seeing a certain puzzle.
- Allow users to upload their own pictures to use as puzzles. But for obvious reasons, don't make these public.
- The auto-rotation, though convenient, kinda took away from the jigsaw puzzle experience. Maybe make it rotate to a random perpendicular orientation on the first touch and allow manual 90° rotations afterward by tapping it. To make these operations mutually exclusive, add a spacial threshold and a temporal threshold such that if neither has been surpassed during an active operation, then handle the operation as a tap on release, and if either is surpassed during an operation, then handle the operation as a drag.
- Add a temporal leaderboard. Only allow times for first attempts. To deter cheating, maybe make it available to logged in users only. Though that's not a perfect solution for determent of cheating, it's at least something. The RNG would need to be seeded for fairness.
- I'm glad to see that you're planning on implementing multiplayer. That'll be exciting!
- Allow bulk movement of unconnected pieces—something like Photoshop's smudge tool. If you're unfamiliar with that, you essentially set the radius of the brush, and press and drag to push anything that intersects the brush circle in the direction that the cursor last moved. Or do something simpler like dragging a rectangle over some region to select all pieces that are contained within the region by some percentage (e.g. 50%) and then dragging the selection to the desired region. The current workaround is to zoom or pan and then move each piece individually to the desired region.
I did the demo puzzle on easy mode, so I apologize if some of this is already a thing.