I guess a four-engine 767 would do it. That would be nice. It would be a great competitor to the new Airbus A340-500/600. Boeing could call that one the 787. The 787 would have the exact same cockpit windshield, fuselage width and cabin layout than the 767 but the wings would ge larger because they would each have to hold 2 engines and carry more fuel to meet the design range. The market for that aircraft would be the Pacific Rim.
I guess one other think engineers will add is a center landing gear in the middle of the fuselage, just between the two main landing gears, like on the DC-10-30/MD-11 and Airbus A340. The advantage that it would have over the 777 on oceanic flights is that no ETOPS certification would apply.
If Boeing starts thinking of it now, it could come out of the Everett plant in the mid 2000's and enter first airline service in or 2007 or 2008.
The other one that Boeing should start thinking of is the 767-500, to replace the 200 which is today (1998) a decade and a half old. Like I explained in other posts, the 767-500 would have the same fuselage as the 200 but wings and fuel capacity of the 400. That would be a good airplane for American.