Another approach might be to ask if there is a non-constant polynomial all of whose positive values (as the variables range in the set of non-negative integers) are all primes. Matijasevic showed this was possible in 1971 [Matijasevic71], and in 1976 Jones, Sato, Wada and Wiens gave the following explicit example of such a polynomial with 26 variables (and degree 25).
(k+2){1 [wz+h+jq]2 [(gk+2g+k+1)(h+j)+hz]2 [2n+p+q+ze]2 [16(k+1)3(k+2)(n+1)2+1f2]2 [e3(e+2)(a+1)2+1o2]2 [(a21)y2+1x2]2 [16r2y4(a21)+1u2]2 [((a+u2(u2a))2 1)(n+4dy)2 + 1 (x+cu)2]2 [n+l+vy]2 [(a21)l2+1m2]2 [ai+k+1li]2 [p+l(an1)+b(2an+2an22n2)m]2 [q+y(ap1)+s(2ap+2ap22p2)x]2 [z+pl(ap)+t(2app21)pm]2}
(From the web page http://primes.utm.edu/glossary/page.php/MatijasevicPoly.html . You can find them broken out here at MathWorld.)
primes.utm.edu . . . nice site, thanks. Bookmarked.