I agree with the mobs suggestion. That'd be a nice feature. Landscape is also a good idea, maybe volcanoes and mountains.
You should also make more, such as farmer/lumberjack's dimension, with trees and such. Maybe also fisher's dimension.
You can also use pistons to move the ores (Albeit it is quite tedious, a normal sized vein can provide enough for a portal), and the fact that they are way easier to get than Silk Touch, it makes it a bit overpowered, since it provides easy diamonds.
... coal is good balancer because you have to first get a book,4 obsidian,2 diamonds,lots of xp to try to get silk touch on your pickaxe or have to work very hard to get pistons to push coal fro ma vein to push it into place and you wont be able to select plac of portal hen and to make it esier you would have to get SLIMEBALLS first to make sitcky pistons so then its easier all in all its balanced cuz it hard to get to thereyou have to get to redstone atleast to be able to make one and if you start there as i said before ou n be able to choose portals place that is the reason why it is balanced (this is for those noobs who odnt read the post)
When I read this, I kind of expected to have it spawn in the middle of a Nether type area that was pure Smoothstone
(So, you spawn in a small box between two layers of bedrock that's full of stone), with less ores(less OP, basically), but with the ores spawning on all 128 layers.
Requiring Silk Touch is somewhat genius, though. Love the general idea as well.
(Diamond Ore would be a better trade off, imo. After all, you trade Diamonds to make Diamonds... And everything else.)