We are not really using the features from simpy (for example, mosaik has its own concept of time); we’re just relying on its implementation of asynchronous execution. Nowadays, the standard solution for this in Python is asyncio. Removing simpy will also necessitate removing simpy.io. This will hopefully make mosaik's communation layer clearer and remove the limitation on package size. (I think there are also parts in mosaik-api, but it might be sensible to move those to mosaik core and only keep the receiving end in mosaik-api.)