Well, Ryan Laukat has done the art for a card from every expansion (except a Promo), and did the new Base Cards Treasures. Similarly, Matthias Catrein has done at least one card from every large set (most of the Base set), and been otherwise involved in every small set (Potion for Alchemy, cover for Guilds, layout for Cornucopia), as well as doing a Promo. I think it's safe to say we'll be seeing at least one card by each of them in Adventures.
Other likely ones:
Alayna Lemmer (done at least 2 cards every set since Cornucopia)
Claus Stephan (done a few, in all except Cornucopia and Guilds, one of Donald X.'s favorite artists)
Harald Lieske (done a few)
Jason Slavin (Donald X. has singled out his IGG artwork as awesome)
Kurt Miller (his CGI monstrosities have plagued us since Cornucopia)
Marcel-André Casasola Merkle (one of Donald X.'s favorite artists)
Lynell Ingram (Maura Kalusky seems gone for good, but Ms. Ingram seems to be carrying the cartoon torch)