I had a Moto Z for 6 months (until it broke), and the Moto Mods dev kit. Moto Mods cover the back, minus a cut out for the camera. The official mods include an optical zoom camera, a projector, a gamepad, a vehicle dock, speakers, power packs, and a 360° camera. There’s a photo printer in the works
Mods in protoype/indiegogo stages include a slide out keyboard, a walkie talkie, a use-it-as-a-computer dock, solar charger, air quality sensor… With possible exception of the keyboard I don’t expect any of those to ship. Moto Mods have the disadvantage that the shape of the phone and the mod are tightly coupled. A mod only works with a Moto Z series and vice versa.
The Essential clip on thingy has a small but non-zero chance of breaking out as a platform. Essential’s design is a) inherently more standardised (just wireless USB + power pins + magnets, b) not tied to the phone’s shape/design. However it’s a tiny market. Unless a third party like LG adopts the standard, then only hobbyists and Essential themselves will care enough to make/sell accessories.
Ultimately I expect USB-C and Bluetooth will be good enough for 80% of people. Particularly if more phones ship with USB alt modes such as DisplayPort - that’s how Samsung’s Dex Dock works. I will miss my Z though.