1. Allow a user to paste in credentials and test to ensure the credentials are correct without having to save and publish the workflow.
2. Have a query that pulls in the Salesforce objects available based on the permissions from #1.
3. Display the objects' fields based on the object type selected

Perhaps create a unique designer area for managing Salesforce data. Most users who may need to interact with a Salesforce connector may not know how to grab the correct fields and thus not want to build a workflow using the action.