I've been working on https://stacks.camera - it's an idea about overlaying the previous picture when you're taking a photo so you can create a timelapse or animation.
For example, you can scroll through 60 pictures from my window https://stacks.camera/u/ben/89n1HJNT
Most of the challenges are around handling images & rendering, but I've also been playing with Passkey-only authentication which I'm finding really interesting.