According the press release we can expect the anthology to include all the Metal Slug arcade titles, which first appeared in 1996. The anthology will also include many additional surprises, one of which is likely to be that the "Wii controller will be used in a new and unique way that enhances the gameplay." That's according to Ben Herman, President of SNK Playmore USA.
The press release also announces the development of SNK Vs Capcom Card Fighters DS. No release date has been announced for either title.