OpenSim Creations has some pre-made megaregion terrain files: nine-region desert terrain, four-region rolling hills, nine-region bay terrain, four-region mountain lake, nine-region mountain range, four-region marshland, nine-region peninsula, and a four-region dormant volcano island with central lake.
Oops, lost the links when I posted that! Here they are:
http://opensim-creations.com/2...
http://opensim-creations.com/2...
http://opensim-creations.com/2...
http://opensim-creations.com/2...
http://opensim-creations.com/2...
http://opensim-creations.com/2...
http://opensim-creations.com/2...
http://opensim-creations.com/2...