Just bundle the next expansion that grants players 300% EXP gains up to level 60. The bonus exp is only applied to the character's current highest level combat, crafting and gathering class. Could also be a new NPC that gives new players a set of untradeable HQ every 10 levels from 1-50 and ever two levels 50-60.

They'll only have to do each leveling dungeon twice at most while still actually having to play through content. I think an instant leveling type potion would be one of the worst case scenarios as levels 1-50 are essentially the tutorial for the game. After all, there's already a problem with people not understanding how to play their class in DF, and that would only exacerbate the situation.