It was bad enough as it was, and all al-Qaeda needed was to "run up the score", so to speak. Khalid Sheikh Mohammed went to college in the US at a historically black university--it isn't improbable he might've been introduced to Memphis, Tennessee, and the numerous key sites both cultural, with attacks on Beale Street and Graceland able to hit the heart of Americans, along with monumental architecture to be destroyed for media effect--the Memphis Pyramid, in 2001 home of the Memphis Grizzlies NBA team. and economic (bridges across the Mississippi and especially Memphis International, in 2001 a very busy airport for both passengers and especially for cargo, and one which had been already targetted for a terrorist attack in Auburn Calloway's FedEx Flight 705 hijacking.
As for the capital, the easiest and most deadly attack al-Qaeda could have done in the Capital area was doing an OKC-style bombing one of the airports in DC or Baltimore at a busy moment and killing a thousand people easily. These airports are major hubs and destroying one of them would cause huge damage both short term and long-term.
It is very fortunate al-Qaeda did what they did and not any number of plans they might have devised. A worse 9/11 is easy to imagine.