I had a few ideas a while back for engi items. I haven't really given them much thought, though, so don't chew me out for their obvious flaws. The Steam forum link is:
http://forums.steampowered.com/forums/showthread.php?p=13962375#post13962375Launch PadSwapped out for the Teleporter, the Launch Pad works like a gravity lift. You can place two, and it launches players or items (weapons from dead players, demo pills/stickies) forward (horizontally), at a 45-degree angle, or straight up. When horizontal, it gives players a Targe-like charge, which could be hilarious, but obviously imbalanced. When angled, it gives classes like Pyro and Heavy an offensive jump forward and up, or it could serve as a defensive/fleeing mechanism to reach platforms/health/metal. When vertical, it lets players reach high platforms, which could be good for Engineers and snipers. The Launch Pad has diminishing returns on "heavier" classes, reducing the distance a Heavy can travel, and discouraging Soldiers and Demomen from taking it because their jumps are more effective.
Railgun SentryA sniper rifle-equipped sentry with a low rate of fire, but high damage output. Its range is 1.5x that of a regular sentry, and has a damage scale related to distance to the sentry; the farther away you are, the greater the chance of a headshot (and accuracy), but lower damage, while if you get close, it has a higher chance of missing due to its slow turn (making Scouts, Pyros, and speedy Soldiers/Demos effective counters), but also higher damage. Really annoying, but not overpowered due to its weakness to close range.
LandminesReplacing the pistol, the landmines are like sticky bombs in that you can only have a certain amount (3-5?) on the ground at any time. They explode and deal 75-100 damage in a small area when stepped on. However, a pyro can disable them by flaming ahead of themselves (effectively making them immune to landmines). The Engineer must "build" each one, which takes one or two full seconds. Soldiers and Demomen can detonate them with their respective explosives as well. Engineer "Landmine-jumping" should not be allowed for balance, but may be possible only when the Engi is ubercharged.
Backpack DispenserReplacing the sedentary dispenser, the engineer can opt to build a backpack dispenser, which has an overall lower rate of replenishment (both health and metal), but can be upgraded once (spend 200 metal all at once, and the backpack upgrades over 10 seconds). Obvious advantages for roaming engineers, but limits the effectiveness of sentries built by this engie due to his roaming tendencies.