Perhaps, but the reason why I think they are omitted from later Metal Slug games is because most sprites in Metal Slug 4 are edited versions of older Metal Slug sprites.
Nadia and Trevor has the same moves and sprites as Tarma and Eri respectively.
The MS 4 team only changed the hairdo and recoloured the sprites.
Still Nadia & Trevor are really cool characters in my book.
I failed to notice any Wifi features btw.
Darn, it would be awesome to play co-op online or online leaderboards.
That would really up the ante for this installment.