Hrm...Traps do seem like a tricky concept to make work in ETG, considering visability issues...
Maybe, something like this could work:
Shadowstalker
5|7 
3\2 | 5\2
Does not cost anything if played under a cloak.
Basically, if cloak is out, this guy can appear for free, but otherwise, his cost-power ratio is pretty terrible. I figure when played under cloak, all your opponent sees is that he's suddenly taking much more damage, but no obvious change to his quanta pool.
I can see a simple solution: traps only cost quanta when they're activated, and aren't shown as being played from the hand. Perhaps they cost HP if the user doesn't have the required quanta on hand, or won't activate without taking the quanta. This also makes sanctuary either very synergetic with traps by making them free, or very unsynergetic by preventing them/forcing a harsher cost.
It seems like we're beginning to talk more about conditional casting costs than autocasting in response, specifically of two types:
1) Cards that cost less if another card is present.
2) Cards that if played at a certain time, change their cost or effect to help you more.
This post will discuss point #1. (I will try to post about point #2 at a later point in this thread.)
Generally, card specific combos limit a card from being useful in most other situations. While it can work, explicit card combos tend to be somewhat narrow, even more so when they directly state the 'combo' card on the ability text. (I would probably pick Vampire or Gargoyle over Shadowstalker in most situations.)
One way to make Shadowstalker more flexible than a 1-combo strategy is to broaden it's available targets and split the cloak into a separate effect, like:
Shadowstalker
5|7
3\2 | 5\2
Affinity : Cost is lowered by 1 for each generating card you control.
Shadowstalker is cloaked as long as you have more than 10 .
How else could we allow Shadowstalker to interact with more cards ingame while retaining the theme?
