Specify event broker service schema.org
Specify a broker server with modules to aggregate events and map overlays in different FOSS platforms.
This handles https://schema.org/Event compliant Microdata/RDFa/JSON-LD, iCal, microformats like h-event, contributed application specific custom protocols. Should have some logic and/or admin interface to deduplicate same event from multiple sources based on e.g. URL. Not sure you want a portal, or just reuse this date in a decentralised way (e.g. every node is able to show the full data, instead of just its own contributions - so people use the portal of their local provider not a consolidated one).
Allow to set labels and filters.