I think this feature is very important!
Examples:

• Scale the mobs differently in every world (increasingDistance, timeFactor, xpFactor, etc.)
• If you add #11 you could make the nether/end or other custom dimensions a little more difficult, forcing players to really prepare for it as the difficulty is already high.
• If you add #6 it could be a nice feature to start leveling mobs where the end islands start generating (the space in between the main End island and the End islands it's too big but "distanceFactor" doesn't care about that).
• Have a crazy dimension with increased random factor, it'd be fun!

That is actually a really good idea, just don't know yet how to implement this

Now added per dimension difficulty with fe27e47
For more info check out the github readme