TL : DR The characters ability to climb a gradient should be a skill.
Due to the fickle nature of RNG I've spent a quite a bit of time in the desert. The mesas look great but I was expecting them to be a greater hazard to navigation than they are. This is largely because my character seems to have the rock-climbing ability of a Skyrim horse. Limiting the characters ability to scale damn near vertical surfaces would make a mesas into interesting navigational hazard with known safe passages and one-way routes instead of a really large set of stairs.
I know this all has to do with colliders and physics materials so it may not be quite so simple to implement but hear me out.
A low level character should have to find a reasonably easy route across a mesa. There are plenty of shallow routes up each of them. If a character practices climbing they should then be able to climb increasingly steeper gradients which, in turn, will open up more routes over the mesa.
This will also benefit tactical assaults in PVP. A sniper with a high climbing ability can use the mesa to great advantage if the enemy cannot get to them by the shortest route.