>>2279329Senketsu's second eye only opened after absorbing an obscene amount of life fibers, so I think that was a one-shot deal (I would even argue that started some type of death-clock in him to ensure he didn't become too powerful). So if he did come back I think one eye would still be closed. I'm sure he will appear in some way since I think a part of the plot will be Ryūko trying to cope.
Although if you wanted to have an ambiguous ending then I would do something along the lines of ending scene with a zoom out shot showing Earth, and then showing a small clump of Life Fibers forming together into a red cloth and then right before the episode ends, the Senketsu's eye opens. Then cut to black/ credits.
