Dear Fabian,
I have a suggestion that should be feasible considering the amount of work you can dedicate and the experience level.
Token simulation is a project allowing to visualise how tokens flow through a process diagram. So far the token simulation provides a user interface to manually halt & proceed tokens and to switch among the sequence flows going out of a gateway.
The token simulation module also provides an API for automatic playback. See here for more details:
It would be a great contribution to build upon this API, to facilitate the automatic playback of a user-provided event log (e.g. as a JSON-file) to visualise process execution for particular scenarios.
With such a project you don’t need to deep-dive into bpmn-js itself, so the complexity would be limited to understanding the token simulation module.