BeamNG is a flexible car game engine for many uses. That's why there are so many aesthetic concepts used at the same time that kinda work. I see nothing wrong with a realistic crazy taxi game.
i think it would be pretty easily done, it could be done almost the same way the christmas snowman thing was done. but adjust the code so that its from one location to another and etc. add a timer with that and maybe something that rates your driving as well for more score.
I think taxi missions would work.. It's just a pick-up and delivery mission and there's already a bunch of those. The hard part would be premise.. If all your doing is driving around, why not just drive around in freeroam? You'd have to earn money to fix the car and buy parts or something. With no real goal there'd be very little interest in playing it...