Title says all. Sign here if you're willing to pay $30 to have an additional character slot. If Bungie will do this then you can have as many toons as you can afford!
Edit 1: It seems like 1/4 of voters says yes. It confirms that there is a market willing to pay for this.
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4th slot could = pay to rng to win meaning you get an extra chance at loot from activities which can easily put you ahead of others. It not gonna happen unless its free.