There is a couple of ways to add vehicles, but part of the charm of Monria, is that the community "feel" here is reminiscent of the 2000 era of Entropia, where MA actually ran votes and polls over features to add or focus development on, and had a lot of community interaction and discussions.. and "TP runs" emphasis on "run" were common, and you had to walk and run everywhere.
But back to topic at hand. Vehicles can be added to Monria, the Entropia platform allows a zone-within-zone defnintion with the appropriate flags. Like the race track on CLD (foma whatever they call it now) but to what purpose?
For example they could add a new crater which is bigger than the other three, and vehicle enabled.
This scenario has a major issue tho - the Role play aspect - all the craters are covered by an air tight dome. Technically all of Monria is indoors. Why use vehicles. But then you can use vehicles in the space station including cars and such.
There is also a technical issue - adding vehicles requires changing the region profile, which gives you the more abstract (and unforgiving) outdoor mining logic. They could create a no mining zone; but that gives the zone little profit potential, so MA will never do it.
There is also a limitation issue - flying vehicles could over fly the map bounds.. there is a no fly zone flag, but on Calypso that is often annoying and breaks the immersion.
It is possible to glitch vehicles in, one player did a total fly over video of the outside dome part of FOMA years ago showing just how lazy MA was designing it.
Another approach is the planetary landing configuration. A special crater could be allocated as the "reentry point" like when you fly to Caly and always appear over the same Mall once you leave space. This would require someone making some sort of space port landing area, which requires time an effort, neither of which MA is well known for. This would allow any vehicle, and maybe even mining. However this also has the problem that it applies the landing fee mode, which makes going to and from Monria cost a "fee". Monria was brialliantly busy early on, often 50+ players there at once, then MA thought it would be hilarious to enable teleporter fees on the Monria teleporter one VU; the result was a 98% drop in income to the Monria revenue, and Monria never recovered from the impetus lost as a result. Most capital ships canceled regular flights here, and nobody bothered to fly there solo anymore. Many new players went to the space station and discovered they couldnt afford to teleport back and rage quit too, which decimated the new player population as well.
Unless they add a whole lot more land to Monria, vehicles sadly will be unlikely.