I need a suitable sized project to implement in a niche language I am learning. I used to use tetris but it isn't big enough. It needs to exercise multi threade

I need a suitable sized project to implement in a niche language I am learning. I used to use tetris but it isn't big enough. It needs to exercise multi threaded updates of some state consisting of many elements and their relationships based on user or file inputs. I think. I want to write it in c++ then learn Haskell and use that. Then use c++ again and look at the differences.