I wouldn't like to queue them up though, would kinda ruin the fun of customizing your gear if you have to wait for it like in a sh**ty facebook game.

As Valkr said a Traditional crafting system just wouldnt work with a game set in 40k like this. That being said an element should still exist. The nice thing about having a class tree is you can have those people who want to Craft/Create/Build things, play the roles who can do this. While those other players who are staying around for the slaying and general destrution dont have to worry about creating new wepons and going mining in order to kill things.
SM players will have those who want to play as Techmarines who will be in demand to create and repair items and weapons in the battlefield. This is how is should be, not everyone in the Chapter is a Techmarine; they are rare even Space Marines (Iron hands excluded of course). Same goes for the Meks, Bonesingers, and Warp-Smiths of the other races. People who want to and see the value in being a support role (who will kick some serious ass on the Battlefield!) will do so, but it shouldnt be mandatory or even really define the outcome of a battle (tank and fortress battles excluded)

Devs anybody?

I can imagine crafting working the way it does in Neverwinter.

This is how it works there: you don't actually craft yourself but recruit people and tell them to find stuff and what to build, but ressources can be found by the pc too.

I guess the "constructing classes" (Techmarines, etc.) could do some things themselves. Question is, how do you balance that one out.

in my opinion crafting system does not have a place in such a game, it would only brake the authenticity and emerjen , you never hear of an astartes looking for 5 iron tails and 3 wolf fangs to make his lance cannon, an upgrade system when you find a rare artifact with psychic powers and you some how attach it to you bolter could be interesring

I like this idea, adds to the Command factor of the game if your leaders in the Chapter can issue missions for people to aquire things. This could include anything from wrecked Vehicles on a battlefield to the control of a mine or city that automaticlly adds rescources to your war effort overtime as the populos works for your cause.
As for the "construction class" why not keep them exactly that? they can reapir and reinforce but dont do much in the way of "crafting". Building walls, trenches, turrets, rebuilding and repairing but not going over to some anvil and crafting four metal bars with three nails and two cow pies to make a lvl 2 Bolter.

Let's hope a Dev will explain!

Maybe we will have some slaves who can craft when we are having fun on battlefield?

I would rather do without another crafting system and all the spam it generates.Getting what I need through pvp is more fun.

This would be perfect. Few worlds in the 40k universe are not populated one way or another and what better way to get the fabracation of materials out of the way then indidgious populations or slave races?
Orks take over an area? NPC orks and gretchins start makin it more ork'y! Chaos insidiously takes over a city? NPC cultests and slaves start boiling bullets in blood and building walls and defenses! Want to help speed up the process? SM Techmarine joins local Adeptus Mechcanicus to create a fortress faster!