Okay, lets stop an think this idea through people.
The engineer is designed as a defensive area denial class. With that said, he can be offensive, but it is very difficult for him to. So what happens then? Most engineers just sit around their stuff waiting for a spy to inevitably come and try to sap it. As stated by Valve, it is difficult to get away from this since the engineer is heavily reliant on his base to keep him alive. His primary weapon is the sentry gun. Once it gets destroyed by a strong push by the other team, how many times does he stay alive? Not many. So, the sole question Valve is facing is this:
How do we make the engineer more mobile, and thus, more fun to play? Most people cannot stand to be an engi because it is so boring. When you try to leave, your base gets destroyed. If you stay, you will become bored out of your skull.
The idea that this thread is based on has many pros and cons, but I will just state a few big ones:
Pros:
1) It makes the engineer much more versatile:
If you are on a payload map, and you are the only engineer, you will always face a significant problem, what to do if my stuff gets destroyed? This solves that problem by letting you build a level 2 base at the front, and a level one you can upgrade to level 2 or three if the first base gets destroyed. Some may see this as overpowered, but it really isn't. If you can kill the engi and then destroy his level 1 base before he can get to it, it is really only a minor inconvenience to your push.
2) It make the class much more fun to play:
This idea would allow you to set up traps for the enemy. You can have a decoy base out in the open, but covered by another base. It would allow you to be much more tactical with your building and thus make the game more fun.
Cons:
1) It makes your bases much easier to destroy:
A level one sentry is really not a problem for a heavy/soldier with a medic to take out. It does not protect you as well as the level 3 does, and it does not last as long. It diminishes the importance of area denial that a level 3 provides. You may say that you can still build a level 3, but a good number of people will try to cover the entire battlefield with three level ones, and thus do the job of three engis. They will fail completely at this, and then start telling people how bad the engineer is. More people will not play engineer because of this.
Overall, this idea is not very good. It has a lot of advantages, but too many disadvantages. It would waste time and effort that would be better used making something better. Sure you can build three sentries, but how many people will do that when one sniper can take out all of them from afar? The pros are outdone by one really big con: It wastes an upgrade. Do I have any other ideas? Not yet, but why does it matter? The engineer's buildings are good enough, and do not need changing. They fulfill the requirement for area denial just fine, and when supported and protected by the rest of the team, can shut down the opposing force completely. The engineer needs better weapons, not buildings.