I would think that EASA would use their own test pilots to determine if the aircraft was safe to return to service and their input would be sought as to the training required for pilots converting onto the type. For any fix from Boeing to be acceptable it's certain to require the pilots to have detailed understanding and specific training regarding the handling characteristics and systems used to protect the aircraft. Previously pilots weren't told of the tendency for the nose to pitch up or the system installed to correct for it. All possible failure cases will need to be provided for with specific procedures and warnings.
Because of the two disasters, any solution will have to go beyond what would normally be considered acceptable for a variation of a current model, and may well require the MAX to be treated as a new aircraft requiring a separate type rating .