My idea has to do with servers being able to sort of 'flavor' themselves as distinct in the Minecraftverse.
The basic idea would be to leave 10-20 block values, the purpose of which is to provide the server with custom, sever-specific block "types" for players to use - both in look and feel and also in quality of world.
The idea would be to assign each of the 20 blocks a set of values by variables in a settings file that would be transmitted to the player upon connection, along with the textures to apply to the custom blocks (meaning the server would transmit the png file to the client to store in memory while they are connected). this file could stay reasonable small.
Some ideas for block-variables:
Hardness -- (what level item you need to remove the block)
Durability -- (how much time you have to spend picking/shoveling to remove it)
Destructibility -- (does it turn into another block type upon destruction, shatter into lava, become water, or simply cease to exist like glass does?)
Sound Effect call properties for walking on / picking -- (Walking and picking)
Gravity Effect -- (does it slip like gravel, or does it stick like dirt?)
Light Transmission Value / Opacity -- (semi-transparent like ice, full like glass, perhaps even a block that gives some amount of like, like glowing redstone -- could be great for making sunlamp blocks for lighting large spaces)
Flammability -- does it burn? does it ignite if near a fuel source? Does it extinguish things next to it or soak up water (something) that soaks up lava would be so nice for building accidents)?
Solidity -- Can it be walked through, or does it block your way and require a jump?
One or two animated blocks in there would be a total bonus, for example: A dimly lit block for creating lighted flooring or phosphorescence in caves.
a custom animated block could give a little light and be something like fireflies for placing in reeds for mood-setting.
heavy hedges or bushes, additional "plants" or custom objects, furniture pieces, could be created.
"wood paneled stone" -- all the beauty of building with wood with no flammability!
a custom textured "sign" or two, or half-block clone would be awesome, too, as those are super handy for creating detail work like furniture.
The big issue with these custom blocks, of course is -- how to get them into people's hands without the /give command.