Anyone using/used https://getindico.io/ ? It is an open source event booking system that CERN uses, and we have a potential client who might want to use this in conjunction with a Plone site.
Hadn’t heard of it.
How many events and how many participants is your client planning to manage?
I helped create an event management system using Plone, which would require no separate integration with Plone 
It used subsites and allowed for self registration or managed registration, speaker profiles, uploadable presentation materials, presentations, training classes. Based on content types we had for 2017.ploneconf.org
indico is something they already use, and are wanting to continue using - apologies my wording was unclear. They are connected to CERN so they are going to want to continue using indico. But your solution sounds interesting; will keep in mind for future pitches.
If there is a big enough user base, it would be strategic to have an available integration.
For sure! and might be good to contact indico "open-source to open-source" 