Vehicles definitely seem like the "E" mechanic (if you imagine "E" being "empty", where you need mana to fuel them to move).
I could see that being something they wanted to put into original Mirrodin too, considering a lot of the stuff in those sets could have been vehicles.
Also the idea is intriguing and I can provide examples of what could have been vehicles from all three sets (including Kaladesh).
If you imagine the art in these commissioned when they were still prototyping vehicles, you can see it.
Mirrodin Potential Vehicles:
Darksteel Potential Vehicles
Kaladesh Potential Vehicles
These are all just from the Kaladesh intro art. Interesting to note that Scars block has no artifacts that look at all like they're rideable and/or hint at Vehicles in their gameplay text. The only thing close is the mech suit Equipment.