I've always read they promised the first two editions to be compatible so I'm thinking they will make the 3rd edition something new so we have to buy more mods. But it would be cool if they keep all generations compatible.
Moto announced a while back that they would support 3 years of Moto Mods compatibility. So to me that means a Z3 lineup and all Moto Mods from all 3 years will be compatible on all 3 series of phones. Sounds great to me, my Mods now will still work into 2019.