As an engineer I've always heard that this is the "PvP profession" and therefore the red-headed stepchild profession as well, since the PvP aspects get nerfed. I think blizzard could embrace the PvP aspect of engineering with regards to wintergrasp by allowing the engineers to make some money selling to players, adding new facets to wintergraps, and doing what engineering does best (on purpose (goblin) or on accident(gnome)): kill people. To do this, allow engineers to craft items that only function in wintergrasp, and have a modest cooldown (5 mins) to prevent overuse, but still allow the item to be useful. A few ideas:
anti-tank and anti-personnel mine generators (produces the mines, 5 min cd). This allows you to set a mine that isn't visible to enemy factions (except those that can find traps). anti-tank wouldn't explode if a player ran over it, and vice versa. For those complaining about the difficulty of defending the keep, a coordinated mine laying operation could help in that aspect. blizzard could also add a minesweeping vehicle to the vehicle shops.
Engineers could also make modification kits for cannons. The southern towers are usually ignored during a fight. Allow a modification that can turn the tower cannons into artillery launchers, increasing range (and reload time). thereby making the southern towers a desired piece of territory to own.
Lastly, allow the engineer to sell siege type items relevant to other trade skills. Such as a device that would allow a tailor with some cloth to contruct a net projector, that sprays nets on players until destroyed. An item that would allow a blacksmith to armor plate a siege tank to increase the HPs, etc etc.
I've got a few more ideas, but instead of turning this into a wall of text, here's the tldr part: blizzard could kill two birds with one stone by adding engineering items that only function in wintergrasp, that any player can use that add to the keep siege/defense aspect of WG.