I believe that ACDC will fit in perfctly with this years Wacken. It is ofcourse a great band that would definetly turn an average line-up into a strong line-up.
Why I believe that ACDC can actually be an option is because they will be working on an album in a studio in Vancouver from May to July. Meaning that in the beginning of August they are likely to have a new album and maybe start touring.
Another thing is that one of the founding members of ACDC (Malcolm) has recently left the band due to illness. Therefore there were questions as to whether the band would continue their recordings (which they will). It might thus be possible that Wacken expected to be announcing ACDC as a headliner, but that they decided to wait with their announcement due to the health issues and uncertainties. I think that might be a possible reason as to why no major headliners have been announced so far.