Well the player's area is almost the only thing I already did, that was my primary focus, I tried to mix "little mounds" and more "large-scale" relief (ex : the quite big hill with "crackled" ground on it), should I do even more large scale ?

(speaking about the player's area only)
By large scale elevation, I mean that some areas of the map are higher than others, but you don't notice it except where the two areas meet. So it's not like a mound or hills, it's just that one part of the map is higher.
The lowest elevation should not be the "default" for all areas, but unfortunately because the map editor starts off like that, that's the way everyone makes maps.
Examples of large scale elevation:
TSK 1: The yellow's base in the top right, that entire area is quite high, try un-elevating it in Krom's editor, you'll find it becomes very flat.
TSK 1: Your own base is quite elevated, then it slopes down to the river/ocean. You don't notice it because it's like the ground is just higher there, but as I said try de-elevating it and you'll see what I mean
TSK 7: Your own base is not at ground level, it slopes down near the beach.
TSK 7: The top enemy base is very highly elevated, which creates a nice effect of it being on top of a mountain overlooking the other enemy base.
TSK 8: Below the stonehill in your base is fairly flat, but above it is highly elevated.
This creates a nice effect of natural flowing elevation, not mounds and lumps in the terrain that look artificial. I've never seen a place where all the ground is mostly at the same height, except maybe The Netherlands

For example in most places, the ground past the beach is higher than the ocean/river, and although it's a subtle effect it can really make KaM maps look nice. (take TSK 1 or 7 as examples) Unfortunately a lot of fan made maps most of the ground is at exactly the same level as the ocean/rivers, which isn't at all realistic.