Brennan was thought to have died in witness protection in 2011, but a surprise twist will see him back on screen later this year as Scott McGregor reprises the role for a guest stint.
Neighbours' executive producer Richard Jasek has now disclosed the first details of the storyline, hinting that Kate (Ashleigh Brewer) may not be single when Brennan returns.
Jasek told TV Week: "I can reveal [Brennan] comes back during an eclipse, where a whole lot of things happen - and Kate won't be quite sure if she's really seen him.
"But the big questions will be: why has Brennan returned, and at that stage, who is Kate with? You're going to have to keep watching to find out, but it's definitely going to be interesting."
Jasek (pictured above) also confirmed reports of new back-yard sets for Neighbours and other on-screen changes.
He said: "Neighbours is currently going through a renaissance. So yes, we're building new back-lot sets that will be backyards for some of our houses, which we haven't seen before.
"We'll be seeing some renewal for our old favourite sets, such as Harold's and Charlie's, as well as Paul's house, plus some new spaces while Lassiter's hotel goes through a renewal."
Neighbours returns to Australian screens on January 7 and Channel 5 in the UK on January 14.