They are supposed to be building a third bus loop at MK to accommodate the animation resort and it will also have transportation to/from the other parks rather than sending those buses to TTC. If they do this, perhaps they will design the 3rd loop to work with the new buses. It goes without saying that MK handles more transportation than any other park so just modifying MK would help things quite a bit. Right now they are only being run on the POP and All Star routes from MK I believe.
From an insiders point of view, I think this is fantastic. The bus system is over worked right now, especially when rails/boats are down for one reason or another. They are the workhorse of the WDW transportation system. I don't think a large addition of smaller buses would solve anything because at peak times as it is right now, buses are backed up coming into MK all the way out to CR and down the hill. Each one pulling in has to wait for an assignment, which is dependent on how quickly a bus can load, which is dependent on how quickly ppl get on the bus! The same argument has been made for ferryboats, that we need a 4th one. It wouldn't solve anything because the load/unload process makes it so one FB is already waiting anyway when 3 are running. Adding a 4th would just be pointless, it will not create a larger guest flow/hour without a significant redesign of the queue area and modification of the ferry routes(both of which I would be in favor of!) But nonetheless, the point remains.