As a relatively new player myself, I realize that I need to make efficient choices with my farming and currency (CUP says "hello"). I'm fine spending some cash to get into the game.
But if the designers were to roll out more small purchases of some older outdated characters, they would definitely have more sales, and it wouldn't break the game.
For instance, sell random $5 "grab bags" of 25 low level characters that are useful but nobody has time to farm. I'd be more inclined to spend more money on $5 packs than the $30 packs they offer