User blog comment:HubToo/Massive structures now!/@comment-25612015-20160117215010

Exchanging region files works, I've done it numerous times.

Smoothing region borders from different origins is also do-able. There are lots of software out there that have interactive tools to do it (WorldEdit being the most well known one), and some command line tools too (like BestOfBoth).

If you start with a blank world, you won't be able to smooth region borders because the neighboring regions are not yet generated.

So the smoothing tool will either have to run periodically, or the new region files will have to be inserted only after their neighbors have been generated.

Smoothing will have to be both vertical and horizontal.

Vertical for flattening terrain height differences (either in the new region, its neighbors, or both).

Horizontal to try to match rivers and roads at borders, when there are some.

Only viewable things would have to be processed. Caves could be left unmatched.

It would be best to take biomes, both Minecraft and LOTR mod, into account, though I would have to figure out the IDs for the LOTR ones.