Summery: Apparently the engineer is harder to update and they are experimenting with ideas on how to improve the class without altering the gameplay. One fail idea was the
Repair Node. This little guy was supposed to replace one of the engineer's buildings to repair the buildings while he is gone for a short while to find metal or fight the enemy. There were many problems with this though.
Play-testing the Repair Node showed us one expected, and two (somewhat) unexpected, outcomes. The expected outcome was that bases were far harder to destroy, and turtling became a super effective strategy. Fortunately, this is the kind of problem that can be attacked by turning the correct game design knob, and the Repair Node had a lot of available knobs. We could lower the rate of repair, lower the amount of repair energy, lengthen the vulnerable period, and so forth. We tried several options. One change we made was to add diminishing returns so that two Repair Nodes working together were less than perfectly efficient, and adding a third didn't really help at all.
Despite the design choices we had available, we were never really able to get the Repair Node to feel balanced for the attacking team. TF2 maps tend to be designed with very specific predicted Sentry placement locations and length of Sentry survivability. The Repair Node distorted old favorite maps and made testing new ones more difficult by exaggerating intentional choke points and creating new choke points where they didn't previously exist.
There were also problems with replacing the dispenser and teleporter which either set the attacking team back or it made the engineer more selfish. In the end, Valve decided they were not going to replace a building unless it's an upgrade or similar building.
More Info At:
http://teamfortress.com/
