could you add a 2x2 version of the symmetry center?, my symmetric designs are based on a 2x2 center

with the parameter C, you can move the center to a corner or edge of the "symmetry center tile" (as is described on the info page here on the mod portal). the symmetry center then still is a 1x1 combinator, but it acts like a 1x2 center at the edges or a 2x2 center at the corners :
C : Move the center of symmetry to a corner/edge of the tile.
0 = north edge, 1 = northeast corner, ..., 6 = west edge, 7 = northwest corner.

Oh, ok, thanks