So, I used to run what was known as Idea Contests for a gaming forum. Like Hero Idea Contest(HIC), Item Idea Contest(IIC) and so on so forth.

As we all know, there is a Card Idea subforum. There would be a poll attached to every Idea thread(i think polls are turned off for this forum). Then once a yearly quarter, they would hold a big Idea Contest, and the highest rated whatevers would be the nominees(like top 5-10)
The said nominees would then compete againts each other in a new thread with new votes required, and thus the "best" is chosen.

As such, I think this game could benefit from a similar system.
Now, to encourage good ideas, there should be a prized attached. After all, the company is technically buying the IP, so why not?

For prizes, the author could win 1 of the card and 50 Gold when it is implemented(contest or not).