Engineer-Update-As-I-Would-Like-To-See-It-V2:
A magnet-operated gun that shoots radioactive railroad spikes. Useful for marking targets, but needs to be reloaded after each shot and requires 10 metal for each shot.
A six-shooter able to destroy sappers in three shots. Long reload - so even with full clip, you can only really destroy two sappers.
An Astro-Wrench, the product of space age (and suspiciously similar to Omniwrench of Ratchet & Clank's fame - yay for thinly-veiled references). Allows you to construct buildings twice as fast as with normal wrench. Upgrades buildings by 50 metal per strike, but won't work if you have less than 50. Deals only half of melee damage, but critical hits still have full power.
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A sniper Sentry, as i said. Has slower rotation, smaller cone of instant lock-on, and more range, more damaging and durable than Level 1, cannot be upgraded.
A Bunker Shield. Like heavy riot shields, it protects your front and a little on the sides. As tall as Heavy Weapons Guy. Much more durable than Dispenser. Has viewport, but no slot for shooting. Passable by teammates, but not enemy.
A Relocation Cart. Allows you to pick up your Sentry, Dispenser, or Shield, and move them to another place without destroying them. They take certain amount of time to pack up and deploy again, are inactive during the movement (Sentry does not fire, Dispenser does not dispense, Shield lies on a cart instead of standing), and pushing the cart slows Engineer down. Replaces Teleporter kit. It could also disable the Destruction PDA.