Seems like we're sliding towards a development interest in a position/field/AoE based series, then. Does 1 card for each element sound like a good baseline?
1 card per X is a card series trap we should avoid unless we coincidentally arrive at that conclusion as a result of development. If we opt for this kind of project, I would think the specific project would determine the amount and distribution necessary to add the concept. I would make a low estimate of 15 cards added in 2 waves(to gather further playtesting data as Zanz did).
A position matters based series would be looking to implement:
Terrain(minor and major), Auras, Minor AoEs(burst, column, cone?), Tactical Movement, and thing's I haven't thought of. (Might even entail a feature added to creatures in general)
This should be broad enough to grant potential to every element but that does not mean that a thin spread would be a good initial addition. The CIA can cover holes via the promotion of single cards afterwards.
"Define what exactly is taboo in card design and what is merely discouraged" As an avid card designer, I'd love to tackle this project. I'm a bit of a perfectionist when it comes to things, and I don't like to post half baked card ideas
I feel this has been covered already although buried with age. It basically is:
Taboo: Graveyard(Zanz: we can do better)
Severely discouraged: Instant Kill without considering a defense(exceptions exist in game), Multi element costs(due to the necessity for coding change and the card cost to implement fully with 12 elements)
Merely discouraged: Target the untargetable and other X the unXable language abuses(but Shard of Wisdom exists)