When I was in my early 20's, I particularly enjoyed Grand Turismo on the PS1, with the possibility of upgrading the car (add turbo, add torsion bars, change flywheel, etc...).
I think it would be nice to come up with something that allow the player to modify it's machine (like adding a priority flow valve to run a mulcher

, letting the player to route himself his hydraulic to install a thumb, or some other attachments, installing a power unit, painting it of course, etc...) But then, you'll need some mech and hydraulic experts on your team, not just programmers...
I agree with Robert_in_NY, an actual fun "goal" will have to make it interesting. Not everyone will like to play Sim-Backhoe. Makes me remember of an old Nintendo game that was a real-time (yes, real-time) bus driving simulation, where the player had to drive crazy distances, like from New York to San Francisco. Imagine the pleasure!
Not sure the usual "zombie-chasing" scenario would be for everyone

, though I'm sure some would enjoy dozing or mulching a few
FD