I think, that the Dispenser needs to be more like that of TFC's were it could be detonated(which you can already do with any Engi building) and destroy those around it. and also acting as a proximity alert system.
Although, i think that the engineers should have there own key menu, like the voice chat keys, filled with options to build.
One col idea is to add options to the Sentry Gun, allowing it to specialise in a certain thing. ie cause it to swap out its rockets for another set of chain guns, or vica verca. Or cause it to have grenade launchers instead of rockets, or give it flame throwers.
Best thing that is going to happen though, is the upcoming swappable weapons for each class due to the completion of the class achievements.
c'mon Napalm Rocket Launcher!!!