Lots of possibilities, and I am sure that Boeing is exploring each one of them.
I think that Boeing is talking about a totally new airliner here, of course using a part or two from the existing lineup, for the 200/250 seat market. A 777-100 would be too big and heavy to replace the 767 line, although one could say that a 777-100 may have been a better alternative than the 767-400, but that is in the past.
I am betting that the new airliner will be a wide-body, 8 abreast for coach with side-by-side capacity for LD3s in its belly, so that limitation of the 767 series can be put to rest. Likely in 2 lengths, one in the 225 seat category and one in the 275 seat category (smaller is really not necessary, the 739 can take over many missions from the 757), each with 2 range options: medium (3,500 miles - 757 range) and long (6,500 miles - 767ER range);maybe an ultra long-range version(8,000 nm) will follow depending on the Sonic Cruiser.