No idea what they will actually do, but what I would like to see is every monster either drop or have the potential to drop 1 or more tomestones. Have it start out as say 1% chance, with every kill in the dungeon increasing the drop rate. Balance it by changing the amount of tomestones and the drop rate as you see fit. Then keep it so boss fights drop a set amount of tomestones, lower the set amount if necessary.