Bringing the meta-biomes idea from the alpha-forums:
How about creating sub biomes (flora&fauna) to every biome to make them more interesting?
Like special biomes around rivers/seas, because they have direct water access.
Or Mountain sub-biomes for obvious reasons.
if its not too much work you could make it even more complex by creating meta-regions for the soil. (sandy, loamy, rocky, stony etc).
To prevent confusion, I don't mean the upper layer of biomes, we already have that. I mean the soil below. Sure, dry regions are more likely to be sandy below and lush are more likely to be loamy, but there's still much room for variations.
For example at my place in Bavaria, Germany we have very sandy soil which is very good for certain crops.
This way you can't just plant everything in one place behind your walls, so trading would play a much bigger role. Imagine huge caravans(or big ships) traveling to tradeposts importing spices from far away. I'd like to create another thread about this one later.
Also, meta-biomes don't need to be everywhere. They can be huge (invisible) areas below the currend biomes, but don't need to be connected to others. Between them, there's just the regular biome where you can grow/find the stuff, you grow and find everywhere. Lke potatoes, you can plant those in lava and they'll grow I believe.