I noticed in Kingdom Builder that the later players have a noticeable disadvantage if they have a card for the same terrain, because there is usually only one good spot to start with (see the question on initial placements), and that will be taken.
Therefore I have suggested and played with this rule:
During the first round, if you draw a card with a terrain that one of the players before you has had already, you draw again.
I did not play it often enough for it to occur often enough to assess whether this helps, and whether later player still have a too-large disadvantage. Have you tried that or similar rules before, and how did it go?