Javascript CRUD ideas feasible in 3 days

Good morning guys!

I've been part of a coding bootcamp (just about to wrap up the first month) and we're going to go into our solo project next week. I'm fairly confident in my ability and have come from an IT background and have a well funded Azure account where I can host the server/database, so scalability/throughput shouldn't be a problem.

I want to build a fun solo project that 30+ people can interact together simultaneously. What ideas (such as a game) can be built out in 3 days with a working knowledge of React/Redux/Node/Express?

I've done some googling and while I could simply do some kind of blog, eCommerce site, or project management app, I don't know that 3 days will be enough time since I'd have to build out the front and back end of the app.

I'm open to using any frameworks that can expedite the process (looking at Backbone.js due to it's model-view setup and tying that in with React since I don't have time to learn Svelte before the project is due)

Thanks!