If I was Moffat I would save it for the finale and bring them back for a big storyline where Doctor and Clara need all the help they can get. Captain Jack Harkness has not been seen since 01/01/2010 TEOT 2 however he has been briefly referenced in TWORS and DOTD. 2015 is his 10th anniversary too. JB has always seemed keen to reprise the role. Plus he has worked with AK on Arrow so they know each other. 12 has already seen some of his other old friends Vastra, Jenny, Strax Kate Stewart and the tragic Osgood
. Why wouldn't he eventually meet Jack and or River ?. In the whoniverse anything is possible even being dead or stuck in another dimension hasn't prevented other characters coming back. In DB said he wanted to put past mistakes right in LC said he got a second chance with Clara. Why not give him time with Jack and or River to do the same ?.Rivers story was suppose to be wrapped up in NOTD but she was briefly mentioned in 8.6 TC. River is not just the doctors wife or Amy and Rory's daughter. Being a part time lord with 2 degrees in archaeology give her knowledge and perspective that might be very helpful to her husband. Especially if it all kicks of with Galifrey and or Missy. Ladies and Gentlemen of the jury the defence rests. 
. Why wouldn't he eventually meet Jack and or River ?. In the whoniverse anything is possible even being dead or stuck in another dimension hasn't prevented other characters coming back. In DB said he wanted to put past mistakes right in LC said he got a second chance with Clara. Why not give him time with Jack and or River to do the same ?.Rivers story was suppose to be wrapped up in NOTD but she was briefly mentioned in 8.6 TC. River is not just the doctors wife or Amy and Rory's daughter. Being a part time lord with 2 degrees in archaeology give her knowledge and perspective that might be very helpful to her husband. Especially if it all kicks of with Galifrey and or Missy. Ladies and Gentlemen of the jury the defence rests. 



