A simplicial scheme is a simplicial object in a category of schemes.
homotopy theory of simplicial schemes is discussed in
David A. Cox, Homotopy theory and simplicial schemes, pdf-slides
David A. Cox, Homotopy theory of simplicial schemes. Compositio Mathematica, 39 no. 3 (1979), p. 263-296 Full text djvu | pdf | Reviews MR 550644 | Zbl 0385.14004 stable URL
étale homotopy of simplicial schemes is discussed in
motives in the context of simplicial schemes are discussed in
V. Voevodsky, Motives over simplicial schemes, Journal of K-Theory, Volume 5, Issue 01 , pp 1 - 38, (preliminary version in K-theory preprint archive: here)
Carlo Mazza, Vladimir Voevodsky, Charles Weibel, Lecture Notes on Motivic Cohomology, http://www.claymath.org/library/monographs/cmim02.pdf