The stealth, as stated, should be used for new players only, the rest will take other things more interesting....or not? It is ok this way, so nothing else should be done.
The current Master Scouts, it is still one of the best traits (I recognize I always pick it).
If we look the old grandfather (MoM), it had Pathfinder (it could be the same: no penalties), and the skills of Foresting (no penalties in forest), Mountaneering (no penalties in hills, and could walk across mountains), swiming (for rivers/water) and flying.Remember that flying units could only be attacked by flying, ranged and magic...Not totally agree with this, but...
I don't remember if swamps got its own skill, or was modified only by Pathfinding. BTW, I think no skill should modify penalty in swamps...
Taking all that into account, you can guess where I'm I heading:
- The most simple: Do nothing...
- The seconed most simple: Master Scouts takes 2 points. It would be like Foresting + Mountaneering (+ swamps for free, if the case...).
- Create the traits Foresting and Mountaneering, to be selectable. And create swimming (for boats, and sea creatures) and flying (for flying units). This seems more for an expansion...