Blockland is missing many bricks Lego has :(
The problem with creating custom brickpacks, e.g. Not rectangles, is that the blb files don't download so the client won't be able to see it or something like that. It'll definately cause issues when you run the server multiplayer, but i can give you the one i've made.
No, because than an issue rises, let's say someone makes a different tree brick, I don't think you will be able to use it.
Also, it will be much easier if Badspot released the bricks.not have people make the dumbest and most pointless bricks.