There's already a first one.
There will be two rounds of DLC available for Samurai Shodown, with the second coming on the date of the game's release.
The first pack, already available for pre-purchase, costs US$19.99 and contains four characters: Basara (from III), Kazuki (IV), Wan-Fu (all games), and Rimururu (II). The second pack, available on the game's February 25 launch day, will include Mina (V, available in February), Sogetsu (IV), and Iroha (VI).
Mina and Iroha were the two most requested characters in a poll taken on Japanese website Dengeki.