If I understood correctly:
Why you need some attribute like required. As suggested you can use .Net inbuild requiredfield validator or You can also make validation on your server side setter of your property. If you find invalid then throw exception or show some message.
If I understood your question like:
You want any intellicense or some help while registering the user control on page it should say required properties to your developer?
Then, I would suggest to have some prepopulatd properties in your usercontrol. Say the moment someone will drag and drop your user control it should pre-populate your properties with some pre-defined values. like
<uc:mycontrol requiredproperty1="someValue" requiredproperty2="someValue1" xmlns:uc="#unknown"></uc:mycontrol>
In this way atleast develepor can see what are the properties required and can change the value according to their requirement.
Hope it helped.