I wouldn't be surprised if Brad Weber takes up that 3rd half back spot. Likely the starter for the Chiefs this year. I think thats why they took TKB on the EOYT no point giving a test to drummond if 2018 is Weber Vs Drummond for that 3rd spot.
In regards to people who may not make it. Someone mentioned Sam Whitelock in amoungst a few names. What indication has he shown that he won't be able. Guy has been a beast without his partner in crime as well.