If anybody who can draw well beyond mspaint then card images would be awesome, but ASCII action shots would be pretty good too. Imagine a hammerdwarf card with a hammerdwrf knocking a creature depending on the deck theme away (Bringing me to a second suggestion, I'll get to that.), like elephants for Boatmurdered, sklek for Nist Akath, gobbos or orcs for soething generic etc.
The deck themes would be based of famous community stories, with decks aimed at replicating the strengths and weaknesses of those forts. A Nist Akath deck would be snow themed with siege engines and a few, highly expensive champion dwarfs. while a Boatmurdered deck would focus on magma based defenses and would be lacking in the offensive department.
A Blockedlance deck would be like a black magic deck, with lots of sacrifices in order to get bonusses in defense and to directly damga the hostile fort (Armok using the power of the sacrifices to nudge the target fort with his small toe, dealing 4 fort points damage, etc.)
An OlonKulet would be another turtle deck, while a Greatbridge or Copperblazes deck would focus on artifacts and megaprojects, etc.
These wouldn't require any custom rules, but it would enable people to assemble themed decks approved by the makers of the game and if possible the story they're based on, that get a few cool combinations.