The first 'phone' (I use that term loosely) with Project Tango was the Lenovo Phab2 Pro. I recommend reading our review if you haven't already, but essentially Tango is the only appealing aspect of the device. It's bulky, not very fast, and expensive.
Motorola's CEO Aymar de Lencquesaing said at a press event today, "We're likely to have a Tango module to basically enable the Z to have Tango functionality." This followed a discussion about how Motorola can bring niche products into the mainstream, and while the Moto Z isn't a wildly popular device, an optional Moto Mod for Tango makes a lot of sense.
We'll have to wait for more concrete details from Motorola, but with the phone recently receiving Android 7.0 and Daydream support, the Moto Z is looking a lot more compelling.