I just pause the game and wanted to share with this idea to You!
Lets's say You have world level 17. You enter any chunk. This chunk have 8 surface squares: My idea is:
Chunks(minimap squares) 1 2 3 4 5 6 7 8 ...and so on.
Monster level: 17 18 19 20 21 22 23 24 ...and so on.
I think this may greatly increase the difficulty even in easiest, because easiest is way to easy
I found cave difficulty system very fun and I hope that same way on the surface may be cool to!
Think about it. You want to search houses for new enchants and gifts etc, but every chunk is the same and entering to each building have a little "linear" feeling. I think it may be more interesting to wander deeply to next chunks with higher shards reward for every monster killed on higher level than the player.
I think You got the point.