I don't generally like making suggestions which greatly change games, but pasik sort of agreed with me in IRC, so I thought why not

This game is very simple at the moment: There's a ton of directions we could go. I would personally love a multiplayer-oriented direction, but the game needs more depth to be succesful there in my opinion.
So let's get started.
Cooperation is all in multiplayer games. Three different classes who must work together to accomplish the victory, would be everything.
Soldier
Basically the soldier we have now. They would only lose the M240, which is engineer's ability to place, and get 1 grenade cap.
The actions, with all classes, would rely on the three mouse buttons, which are easily achievable and are important.
With the soldier they would be simple: LMB to shoot, RMB to strike with the knife.
If secondary weapons are to be added, scroll to change between them.
Soldiers are the backbone of the army, the serial killers, but they do not have any speciality, so they are in a general disadvantage to the other faction if the enemy has all classes in use.
Move along.
Demolition man
Since we have those tanks and jeeps coming, I would imagine they will be quite powerful, and therefore need a good counter.
Demolition man would have a
Bazooka2/3 screen range, to not to be overpowered
Explosion radius as big as the grenade explosion radius
5 rockets, refill on nade boxes
and require crouch to fire.
It could do 90% damage to a tank in a straight hit, and 100% to jeeps / trucks in a straight hit.
Reload would take 2 seconds, to leave the demolition man very vulnerable to enemies, so therefore he would need soldiers to protect him as he'd be quite easy to take out. But tanks would have no problem in taking out soldiers ifiamrite, so it's kind of a rock-paper-scissors situation.
Demo man should also have a pistol, and one C4 (to destroy enemy fortification type of things, to set up traps or to destroy vehicles) which you could trigger after throwing it with RMB.
LMB = Shoot / trigger
RMB = Aim bazooka (crouches and slows down the mouse, improves accuracy) / throw C4
Scroll = Switch weapons.
And now to the most game-changing class suggestion,
Engineer
I have come to realise that building + fighting is the ultimate game when done right.
The engineers would be able to place sandbag walls, barbwires which slow down and hurt the enemy, landmines, hedgehogs (to stop tanks), mountable machine guns, and whatnot.
Each placement requires resource points, let's say one sandbag wall part needs 2 resource points and a MG 5 points.
You would get these points by choosing the wrench from your inventory, and spamming RMB at containers which we already have in the game. This way it'd add a lot strategical priority to which areas you hold.
The containers would have resource limits too, say 5 rp (resource points), gathering one rp would take one click, and one rp would regenerate after 10 seconds in that container. This is needed so that you can't just spam everything everywhere, you'd need to wait at least. Though I don't believe massive wall mazes would be very common anyway

Oh yeah I mentioned wrenches. They would be one tool in the inventory you use to repair vehicles, gather resources, or to kill enemies. It'd do all these things depending on which one of them you're standing next to. Whatever you do, the wrench'll be always swingin' though.
LMB = Shoot
RMB = Place objects / Repair vehicles / Gather objects / Remove objects / Melee
Scroll = Change between objects in the inventory
The sandbag walls and barbwires should be able to be destroyed by grenades since they are a potential griefing tool, say 4 hits to each part would bring them down. Engineers would be able to remove them too, by swinging the wrench next to them. Also maybe when you ride over them with a tank, they'd go BOOM. Not the hedgehogs though. This'd add even more strategy and required acknowledge of the game, see this picture:

A potential stronghold, tanks can't easily get there, nor do enemy players so they can't throw grenades. Surely a tank (engineer also, but I'd imagine they will die swiftly into rapid MG fire) could slowly destroy them all, but that'd take time and enemy demoliton mans would be able to sneak to them and blow the tanks up. Enemy soldiers would prolly be protecting them though

How'd building work?So basically if you want to build a wall, you click and drag the way you want the wall to be. After that your character would automatically goes and builds each part, and the actual building wouldn't take long. The same for barbed wires.
MG's you need to place on walls, and landmines you can place anywhere. It's quite simple.
Generally engineer-type of class would add a lot strategical elements to the game, and lots of the needed depth.
We were discussing this a little with pasik in IRC, and he said he'd make soldiers to have different abilities by what items they are wearing. I however think that making clear different-looking classes would be better, because it would be very hard to identify who's who and who's doing what. ''Is that an engineer coming up or is it a soldier?'' and ''Does that guy have a rocket there?? Maybe I should kill him first :S'' kind of quotes wouldn't be very rare, I suppose.
Without clearly different classes but with clearly different abilities I think this game would become complicated.
I haven't thought much about how to change the classes. Maybe by pressing a certain key you would get a menu where the three different classes are able to be chosen, with kind of big images and small explanation texts. After you will pick one, you could be assigned to that class after your death.
Also it could be a little hard to code the AI to behave properly as an Engineer - so I'd suggest that the bots only pick up Soldier and Demolition Man. Don't wanna see random barbwires covering all your spawn
''What's the point in building a stronghold when enemy can just capture nearby bases??''This is a problem I thought may occur, so I suggest that you'd have a checkbox in the server settings - which as enabled makes only bases which are being targeted, be able to be captured.
Like this there would be a bigger need to build defensive positions, when all enemy forces are being centered to the same and one base.
But before this we need all kinds of basic stuff, such as server browser and chat history.. Yeah well. I've shout my thoughts now.
Click Preview... I should have remembered everything. Feel free to build on the idea if you wish.