So I was messing around with the custom worlds, and I realised that you can type in numbers beyond the slider limits.
This means it is possible to set the biome size to 11, (default 4), which is 128 times the default, and 32 times the 'large biomes' setting.
The biomes are HUGE! As in, walk for 10-20 minecraft days before you reach the edge of the biome, huge.
I have also altered the terrain borrowing from here so the terrain feels much more natural.
In addition to this, to make things interesting, I have been putting diamonds above the clouds so that you have to find a mountainous biome to get diamonds. I still haven't worked out the perfect world yet, but this one is close:
Diamonds are above y=160, so you have to find mountains . I have previously tried y=192, but that was a bit too high. I might try y=128, but also make Lapis much rarer so it is a bit easier to find diamonds, but harder to enchant tools.
Iron, coal and redstone are more common, while gold is more spread out (more ore veins, but less in each)
In my most recent world (seed=-161880275703364513), I spawned in a roofed forest:
spawn region, endless roofed forest:
I have found 3 mansions already, here's the closest one:
nearest other biome, plains, 1500 blocks away:
boundary with a taiga hills biome:
A mountain containing the closest diamonds, over 12000 blocks from the spawn point!:
screenshot of AMIDST zoomed out as far as possible:
I will still play around with the ores a bit before I decide on the perfect world, but the terrain is definitely good.
I think the ideal world would be one where iron and gold are common enough so it is easy to build long rail tracks, but you have to go looking to find diamonds, and get lucky with the Lapis Lazuli before you can get enchanted stuff. It would be an extreme exploration world, with the long term goal of finding all the biome types (in this case, I don't think using a biome mapper like Amidst is cheating since the hard part is getting to the biomes!)
Here is the preset for the huge biomes and terrain with the default ores: