I don't know since I saw the Skyrim new environment complexity I'm tempted to remake the level from zero once again (that would be the 4th time - oh god).
Maybe make it smaller and fill it up rather then large empty spaces.

Gamestudio definitely cannot handle 1 world level so I have to split it in many different regions.

Yes, regions will really do the job.

And don't do like a lot of people targetting all big games made by thousands of people !
Your game counts, not AAA games sold in the market.
Your game will have is own universe, is own system gameplay, is own story.
Like Borderlands, ro even smaller regions with transitions zones (Zelda Twilight princess like) will indeed do the job.

Even lot of pople that like RPG s sais that Xenoblade on WII was one of the last real RPG n, with great story, even on WII : polished graphics in the details and very very good level design and character design are ot better than lot of Today games made without such deep story , such deep study in level design !

So not only thousands of polygons or last shaders make a great game