Hey Anthony, great question.
I think retention is massively important - as the saying goes it's much easier/cheaper to retain a customer than acquire one. Having a loyal customer base that returns again and again to buy tickets is a really strong asset.
I guess beyond making sure the attendee experience at the event itself is great - and making people want to come back for more - communication (email, social, an online forum) is pretty important in the downtime between events to keep people engaged in your brand. Not that i think i've ever achieved this in a measurable way, but having a real community around your events who come back time and again and keep the discussion alive year round i think is an enviable goal for any event organiser.
On a more pragmatic level Facebook retargeting ads are a go to for bring back past attendees and website visitors etc.
This is all online stuff really but would be interested to hear people's offline ways of doing this?
Can i ask what are the key data points you're looking to build into the solution?
This response here might also be helpful: