What are the restrictions preventing more intelligent autopilot systems?
Under the FAA there are not really regulations that limit the ability of an autopilot and shy of takeoff most modern systems are capable of performing most of a full flight with no pilot input (aside from route updates etc). Ultimately any aircraft system will have to get certified and this involves rigorous testing as well as demonstrations the system works under all stated cases. 

There is however a bit of a logic issue in your statement:


  For example, during flight under certain condition where some sensors
  are failed and flight control system reverts back to direct law (in
  case of FBW) and the intelligent system autopilot recognizes this case
  so it helps pilot to decide what to do or controls the aircraft
  thoroughly


The reason the autopilot disengages or disconnects is because, due to the sensor failure, it no longer has the information it needs to properly control the aircraft. No matter how smart the system is, without proper sensor input, positive control is not possible or would be logically unsound. This is even true for human pilots and was in part the issue that lead to the Air France 447 disaster.

There is a broader question here of autopilots potentially handling emergencies which is a bit different than a sensor failure (read pitot static failure). This is already in place in part on some aircraft and as time moves we see more and more of it. Aircraft are now capable of detecting a depressurization and automatically descending to a safe altitude; even new small GA planes have this feature. Autopilots also now have forms of automatic level mode which will bring the aircraft back to a straight and level attitude at the push of a button.

This also comes down in part to design philosophy. This is highly opinionated so I'll stay away from it on this site, but there is hard data to support both cases: in the event of an emergency, is a pilot or computer system better equipped to handle the situation?