a forest nothing world generation
almost flat terrain, only dirt blocks below the surface all the way down to bedrock
all forest biomes spawning, no other biomes
but changed so the trees are as tightly packed as the roofed forest trees in every biome, and bamboo forest is just bamboo on every block, no other trees
villages still have to spawn, since water, lava and many other useful things can only be gained through trading, but all the blocks making up the houses, apart from glass windows, need to be the local wood

how about Something like the timber datapack for the giant mushrooms?

Here is an idea that also fixes an annoying problem: make the games cat cap not effect iron golem spawns. If you build an iron farm, it will spawn cats, and if 10 cats spawn, the village can not spawn golems anymore. On top of that, you have to kill a cat close enough to the village for the villagers to realise a cat has died and they can start making golems again. This is my biggest pet peeve with the game and I would love to see it fixed.