Considering most of the stories this season has loose ends, there is certainly material to fall back on in the climax at the end of the season.
We have the immortal girl, the sandmen, the Zygons, Missy, and the ghosts in the Faraday cage.... all still floating around in the background somewhere, and all feeling unresolved.

Only three episodes left of the season, and the last two are supposed to be a 2-parter... so whatever they do it had better make sense.