Donald has mentioned this before, but more new cards also requires more increased complexity. There would need to be more new mechanics; new rules that take time to understand beyond what's just written on the cards.
Also, more cards could be introduced if there wasn't the concern for making sure you don't make other cards obsolete. Donald has said before that certain cards couldn't exist just because they limit other things (I think he said he once had Throne Room at $3, and Throne Room with a bonus at $4. TR ended up needing to be $4, which required scrapping TRwaB completely, because it was terrible at $5). But, if you weren't concerned about both TR and TRwaB existing at $4, then you can have both. MTG does this, both Shock and Lightning Bolt exist.