Not sure if you mean "engineers in general" or "engineers who specifically work on Lemmy.world", but assuming it's the former, I'm working on building a self-generating webcomic.
It's always been my dream to create a website that can completely self-populate with pointless content, and soon my dream shall be a reality.
I've planned to dive into the authentication side. I think it would be nice to be able to host multiple services in example.com and login on those via 1 login. This way you can post on lemmy and kbin as user@example.com, follow people on mastodon, be reachable on matrix all with the same identity. (And when you want, even receive email)
I doubt it would be useful for open instances, but it could be nice for self hosting.
Building a mid level statistical analysis web app with python, flask and jinja2.
Developing this has been a ton a fun since I get to use my statistical knowledge skills together with my programming skills. So far the feedback from my users has been overwhelmingly good so I am very encouraged to continue working on it.
In all seriousness, the last "cool thing" I worked on was a research project to design medical exoskeletons for a certain class of people with disabilities. Can't go into too much more detail or I'll dox myself.