I have my concerns about a Druid being better thief than a Rogue of the same level. As a DM, should I prevent using the Wild Shape as a high-end burglary tool, and how?
The Wild Shape feature has no restrictions for an animal size, which makes this feature quite tempting for any rogue, from my understanding.
Starting at 2nd level, you can use your action to magically assume the shape of a beast that you have seen before. You can use this feature twice. You regain expended uses when you finish a short or Long Rest.
Your druid level determines the Beasts you can transform into, as shown in the Beast Shapes table. At 2nd level, for example, you can transform into any beast that has a Challenge rating of 1/4 or lower that doesn't have a flying or swimming speed.
PHB gives examples of turning into wolves, crocodiles and eagles. But really overpowered is turning into a very small animals, like spiders or mice:
The greatest thief
At 2nd level, the Druid becomes an ultimate escape artist and completely overshadows the Rogue:
Infiltrate a heavily guarded territory. Forget Stealth checks, making a disguise or bribing guards. Just turn into a little mouse and go through the main entrance. Or you can turn into a spider and climb walls unnoticed.
Pass a reinforced locked door. Again, turn into a spider and climb through a keyhole or under the door. Since all the equipment can be merged into a Wild Shape, you can take your loot with you.
Hide in an empty room. Enemies are about to find you, but you don't know the Invisibility spell? Turn into a mouse! Besides from Stealth checks advantage you will get Total Cover even behind the smallest objects. Or you can turn into (again!) a little spider and just sit on a ceiling.
A ghost that leaves no traces. Pass traps, walk on pressure plates…
Escape from any ropes and chains. Being chained into manacles, you don't need to make DC 20 STR or DEX check. Just turn into a mouse.
Escape from a prison. There are plenty of rats in prisons and dungeons. Turn into one, slip through the cell bars and be free.
As a DM, how can I limit the Wild Shape feature, to make the Rogue on par with the Druid?