Can there exist any element which has an 8th principle shell?
That is certainly possible, and in fact very likely. Nuclear physics predicts an island of stable elements around an atomic number of 120 $^{[1]}$, with the two heavier of the three elements predicted to be most stable having an $\ce{8s^2}$$^{[2]}$ and an $\ce{8s^2 8p^1}$$^{[3]}$ configuration, respectively.