Firstly, I don't think any method likely to be used can match the death toll of 9/11. I don't know of anything other than an large Jet plane crashing into a tall buildings full of people that can kill quite so many of us. Neither a Ryder truck nor a small plan full of explosives is like to kill quite so many people. (If they were the 9/11 hijackers would have used them - since both are easier to carry out. A small plane can be trained for and bought much more easily than large plane flying lessons. A Ryder trcuk requires almost no training.) Give that security is still pretty awful could we have another suicide hijacking? Damn unlikely, because now that people know it is possible suicide hijackers will be taken down by massed passenger -- just as happened to fourth plane that was crashed in Pennsylvania.
There are some possibilties that would be worse than 9/11 -- but they are damn unlikely. One is that Osama gets hold of nukes. I think the argument here is that if he coulda he woulda. I would add that our policy right now, by increasing sympathy for Osama in nuke-armed Pakistan is making it more likely, not less. But more likely is not the same as probably -- and I think (and hope) that this will not happen.
A worse possiblity than nukes would smallpox - available as I understand it only in the U.S. and former Soviet Union. It is very unlikely - but if it happened I think the death toll would exceed WWII. Smallpox is very infectious, and people all over the world travel to and from the U.S. A revival of smallpox in the U.S. would mean a revival worldwide. We might (though I would not count on it) get it other control with only a few hundred thousand U.S. causualties. But it would take decades to get it under control in poor countries (including Afghanistan). I hope Russia and the U.S. are guarding their "defensive" smallbox stocks extremely closely. They probably are.
So leaving aside the worst scare scenarios, what is left? Anthrax, which is not that infectious. Bombing bridges, or oil refineries, or chemical plants. All horrible but with likely death tolls in hundred rather than thousands.
And while the major concentration of our current administration seems to be on limiting civil liberties, it seems likely that eventually they will get around to actually doing something to make life less convenient for peotential U.S. based terrorists.
In short regardless of what we do in the Afghanistan another attack on anything approaching the scale of 9/11 is unlikely. In short, if attacks killing hundreds or even dozens happen in time frame you have described, than by your measure the retaliaion will have failed.
Last point -- although the U.S. has claimed to have killed some Taliban leaders, no evidence has been presented -- just a bare statement, not repeated. We have not even claimed to have killed anyone in the actual terrorist networks. So far the only people we have killed are innoncent civilians, and low level soldiers. As winter comes U.S. capability to kill terrorists and Taliban leaders will decrease, while our cutting off of food suppiles will definitely kill tens of thousands (at least) of Afghan civilians who would otherwise have lived.
After the winter is over, the terrorists networks, and the Taliban leaders will have had plenty of time to move to new hiding places. The number of anti-U.S. terrorists, and their sympathizers and supporters will have grown a great deal more than if we had done nothing in Afghanistan.