'Levels' are interconnected so numeration is irrelevant. You can go from 5 to 10 or from 6 to 13 for example, and 1 can be connected to several others. They won't add a level "in middle of the previous one", obviously ;d Golems level(which is level 6 internally) is connected to 5th, and 4th as well. The doors and connections are already there, just locked. Don't look at it as a linear progression. ;d
And yeah, natural terrain does not mean outdoors stuff ;d The 1st 3 levels will also be upgraded eventually to take advantage of the new system. Adding for example destroyed terrain and what not.