1) Steal keys for coffers, chests, and doors.
2) Steal key items for quests/missions.
3) Steal crafting ingredients for item synthesis.
4) Steal pop times to spawn NM's/HNM's
5) Steal mob parts to defeat a boss (e.g. A Stone mob will not die until you steal his keystone, but first you have to wear him down or kite him through a maze)
Pvp stealing idea:
1) Steal specific item (e.g. thieves crown) from each other.