When Soulkeep went on presale there were suppose to be multiple promo cards released at set levels of sales. Celene, currently the only hero for Soulkeep was the first and only "stretch goal" promo that was unlocked. That doesn't mean that the cards weren't already planned out though as we can see them if we look in the spellbook and sort by All Cards and Legendary.
Here you can find 3 cards with 0 in circulation and listed as having been promo cards when clicked on to examine further, The towers Mana Harvester and Torchy along with a new spell Traitor Maker.
First up is Mana Harvester...
Its name is what it is and if/when released this card will likely become the most played tower in the game. Mana directly = score, and this tower simply generates increasing amounts of mana at faster intervals per tower level. It does no damage or interact with souls. Currently everyone fights over who can most effectively generate mana off the limited number of souls available per map. This tower adds a new mana source entirely. Maps will then come down to who squeezed this in earliest while still managing to optimally kill the souls. I know I'm practically salivating at the hope this will still be making an appearance.
Next up is Torchy...
Torchy is a tesla tower like Searing Vision which locks on 1 target at a time and does slightly more damage per tick. Torchy would be the physical damage version to Searing Vision being magical damage. Comparing the 2 Searing Vision does more damage and fires faster at higher levels. Torchy however can place 4 towers instead of 2 and the lock-on damage ramps up to 2x instead of Searing Vision's 1.1x. Overall I'm not sure how much use this tower would get as I don't believe the tesla attack type is really used much. Especially since at least Searing Vision doesn't attack fliers. If this targeted fliers maybe it would get some use but otherwise I don't see it being used much.
Finally we have Traitor Maker...
This is a potentially interesting spell. As it is leveled the spell targets from 1 - 10 souls and the uses increase from 1 - 4. I don't see a spell duration listed so it appears the spell makes the souls fight for your team till they die. If used competitively or not will come down to the details here. Traitor mobs could end up costing a player mana/score if not killed or could be overall worth more dying in a Mana Mania or to a carefully positioned Voodoo Sapper. However it could also be really fun to see how long your soul army can last and what you can capture. Pixies are often timed to either come out with other souls or arrive together from different paths. Capturing them when they walk out with a Slaying Mantis for example might be interesting. Do the mobs summoned by the Mantis over time fight for you too or immediately turn on their creator. Throw Celene into the mix and she can get heals from the Pixie which could be massive. I'd rather like to see this card come out. Once more though it would depend on how it was implemented.
Any of these cards have your particular attention?