Rather than going for the usual suspects like The Ur-Dragon or Tiamat, why not take a look at a hybrid dragon-aristocrat-reanimator approach
To figure this out, you need to think about what's going to matter in this game as a whole, as well as ask some questions about what you know of your own deck
Soul Sear is another one that does damage at the same time, if you were looking for a starting point to wreck your indestructible friends
The reason is structural rather than emotional: promos are distributed in fixed, predictable quantities through specific channels, which creates both uniform distribution and a powerful perception of special-ness that incentivizes grading
Sulfurous Springs: black or red
I got to thinking about what the prices on the serialized bobbleheads will get to, and then I realized that we need to examine the history of these cards and see where prices might go