I guess the big question is what you might be planning to use a new lens for?

If you do use the long end of the 70-300 enough, and don't feel the need to go longer, then a 300mm prime would be an excellent addition. Use the new version myself. It loves the 1.4x TC. For carrying and handholding, the 300 with TC is a LOT lighter than the the new 400mm. The new 500mm is 1.5 pounds lighter than the 400mm as well if you want even more reach. Might be pushing a lot of peoples limits to handhold for a while though. If a little more budget conscious the 300 f4 is a nice lens as well, and a lot lighter.

Your equipment list doesn't specify whether the 100 mm lens is the macro or not. Is that an area that holds any interest?