I know that required fields are set by not allowing null values, but this is not exactly an easy operation to perform after a layer is created. It's often not a best practice to export a feature class' schema, changing the properties, then importing the features into the new schema. It can change ID's and cause unforeseen issues.
My suggestion is to implement a property in ArcGIS Online that allows the administrator to set field requirements in the application independently of the feature classes properties. Helpful for administrators to specify which fields NEED to be collected in the field using ArcGIS Collector, without having to make adjustments to the feature class' properties.