For years I have shot my panoramas as 6 horizontal shots around, spaced 60 degrees apart, and one zenith shot straight up. However, I rarely bothered with the nadir shot where the tripod stands. The result was a hexagon of nothing, where the horizontals panes met but didn't come together at the nadir.
The easiest way to deal with nadir hole is to cover it with a layer bearing a logo then flatten the image. No need to align anything or tweak colors and such to make things blend naturally, as if no tripod ever existed. The few times I had set the tripod aside to shoot the ground beneath it to fill the nadir hole, I met with failure.
Tonight I took my freehand nadir shots, ran them through LensFix CI to defish them, cut out the section that would cover the hole, and used it in a layer. That layer was resized, rotated, and tweaked with the Warp tool under Edit:Transform. Then the layer's Levels are worked to get as seamless a match with the surrounding background as possible. Flatten a last time.
Surely greater competence with Photoshop would help, but I like what I've done: capped the nadir with a natural look.