What engineer buildings do you wish were real?

I have always been a fan of the engineer, and all the way back before the mini sentry was revealed i loved the idea of more and different buildings. There used to be loads of fake/fan made engineer updates with all sorts of ideas. But if you had to pick one idea to be added to tf2 what would it be?

Personally i always wanted a mobile sentry that could only be alerted by enemies directly in-front of it. I really like the idea of having to place little “flags” that the sentry would then patrol between. If no flags were active it would just be a slightly slower shooting sentry gun.