Physics simulation is imho one of the things that feel like they should be easy but in reality is really difficult. And not talking about anything super fancy, just basic classical mechanics.
One of my dream projects would be to glue physics engine like Project Chrono[1] with modeling kernel like Open CASCADE[2], and some (FOSS) CAD (solvespace[3]?) to make an user friendly package to be able to setup simple scenes for simulation.
[1] https://projectchrono.org/
[2] http://www.opencascade.com/content/open-source-core-technolo...