I have many ideas for this game, as any player who have played. But Im only gonna wright one of them here for starters.

Engineer

Divide game in 2 classes: soldier and engineer. Soldier will be the typical stat we have now. Engineer will be starting with 100 blocks (twice as many as a soldier), 75 health (3/4 of a soldier), 1 grenade (soldiers have 2) and 30 rifle ammo
(soldiers have 50).

Now here you see a typical suggestion posted many times before. Lets go on to what makes it not so typical. Engineers have the ability to craft different materials. We can start off with 3 and as time goes by, game developer may add more.

#1. Natural: the blocks we have now, nothing different.

#2. Concrete: takes 5 natural blocks to craft (player presses, lets say "C" on his keybord just as an example), it takes somewhere between 1 and 10 seconds (up to game developer) to transform 5 natural blocks in to one concrete and the process will go on until the player has no more natural blocks on presses "S" (as an example).
Concrete blocks have the same size and look very much like the natural blocks, but they are actually very different from other blocks:
A) Only engineers can craft them and hence ONLY engineers can use them;
B) They can NOT be destroyed by the shovel (demolishing concrete bridges with a shovel, seriously?) nor that other thing that i do not remember name of (hotkey "2");
C) Concrete blocks can NOT be destroyed by fire from rifles;
D) Concrete blocks CAN be destroyed by grenades and shovels from OTHER ENGINEERS, not from soldiers as I pointed out earlier. It should take a fair amount of time destroy them though, like 3-5 sec per block.

#3. Metal blocks: are crafted in the same way as concrete, except that it takes 10 natural blocks to create one metal and time crafting may also be a little longer. Overall they are the same blocks as concrete, but simply "reinforced":
A) They CAN be destroyed by grenades BUT they take a lot more damage, so it should take about 3-5 grenades to explode ner a metal block in order to destroy it;
B) They CAN be destroyed by OTHER ENGINEERS, but it will take about 10 sec for an engineer to destroy ONE metal block with his working equipment (shovel and stuff).

This is to prevent griefing and make the game more realistic (as I said before, demolishing a concrete bridge with a friggin shovel? Seriously?)