I don't really like this idea, Triple income is just way to much. Everyone will have the max goldmine in a couple minutes and they will be able to purchase every tower and every mob they want. Also like
@Blocky said, Since everyone has everything maxed out it will be harder to reach the castle so the game will take even longer than it already is. Adding an Armageddon mode on the 15th minute sounds good but wouldn't do a lot since that everyone will already have a lot of maxed out towers to defend their castle.