There's only two ways to get the casual viewers back; one is to keep making a great show and hope they catch an episode and get re-hooked, the second is to shake things up, and much as people may or may not want that shake up to involve Moffat the truth is that the casual viewer won't give a shit about the show runner, so that change would have to be the Doctor, which frankly I think would be unfair on Capaldi.
Option 1 is playing the long game, option 2 is the knee jerk, quick win, but the downside is that you can't keep playing that card. I'd hope the BBC would go with option 1, I fear they'd go for option 2...
Maybe a new companion could generate some interest, especially if an old companion returned or someone relatively famous/unexpected got the job.
Well having a consistant time slot can help as well, next weeks episode is on yet again at a different time.
Capaldi seems to be more comfortable in the role this season so far, or perhaps it's the stories are better suited to him or perhaps It's just me. As for change we are due for a companion change shortly so that might shake hings up a bit, but they should go for something a bit different than someone from modern day Earth, if they must have one from the modern day they need a second who is from another time period/alien/robot etc..