Hey guys, about question 4 and 5:
there's an interesting
suggestion (implemented for test in a mod) that introduced the ideia to make
lava different colored in biomes, so it could be considered other liquids.
In this bundle of ideas (courtesy of our fellow
@Snickerbobble), lava in corruption would be lime green as acid, and in Crimson would be gold yellow as ichor.
The modder (our partner
@jordan) that tested the original idea even proved that the lava could behave differently over different biomes (like water do in tundra), with many different status ailments and debuffs. I find this pretty more interesting, designwise, than recoloring water, for many purposes.
Anyways, about trees in Tundra, maybe you don't know, but the game chooses randomly between four types of winter trees to every world. The first is the standard Terraria tree, covered in snow. The second is a leafless tree, with its spiky twigs. The third is a pine tree, without any snow (like you suggested). And the fourth is the pine tree, covered with snow.
So, if i have understood your 5th idea correctly, the game already has it implemented.
#Edit1 - provided links.