With all the issues surrounding Lab Zero Games and the owner Mike Zaimont, after most of the team left and the rest were let go - 505 Games have given a statement.
Their statement concerns Indivisible, since Lab Zero are basically no more the publisher was forced to clarify what will happen to it. In short: it's finished. In the statement on Steam they confirmed that "apart from content that is already in submission, there will unfortunately be no more production on the game". This means the planned Guest Characters and Backer-created characters will no longer be coming.
In a further post on their own website which has an FAQ, it confirmed that Indivisible will continue to be sold and will not be removed from stores. This extra post also mentions that the IP holder for "at least one of the guest characters has backed out and we expect more will follow" - so it seems even if they wanted to do them, they can't.
Such a huge shame, I thought Indivisible was genuinely great and it was one of my highlights from 2019.