I don't think it would be good because we'll be spending all this time upgrading and levelling up our characters and then when a new class comes out that we prefer then we'll really want to play as that character but we'll have to start all over again!
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I could see how you think this is a negative thing but surely it would be better to add a new class someone prefers. Also, the level cap isn't that high anyway so it shouldn't take an awfully long time to level them up. Also, this would give the game more replay value and keep the player interested.