Losing Ellison for most of the season is a big blow for the Highlanders -we will really miss his physicality in the 13 jersey. He is a bit of a loss for the ABs too as he is currently the 2nd choice 12, though a fit Kahui could cover there (a fit Kahui is rare though).
I wouldn't be unhappy to see Smith at 13, but I'm not sure Ensor is quite ready yet to start at Super Rugby level. He had some outstanding games for Otago during the ITM Cup, but did also have a couple of shockers. I would definitely bring Ensor into the main squad to cover for Ellison though, as we still have plenty of midfield cover even with Ellison out (Treeby, Burleigh, Nonu, Emery, Smith - even Poki can be pretty handy at 13). If Smith plays centre I think Slade may be the best option at fullback at the start of the season (for the 1-2 games he is fit). He has probably played his best rugby at fullback, and Sopoaga could be exactly what the Highlanders need at 10 to unleash the rest of the backline. Another option at fullback could be Popoali'i, but I think he is better on the wing (where he has more freedom to roam), and I'm still not convinced about him defensively at Super Rugby level.
If the Highlanders stick with Smith at fullback I'm not sure who the best option is to pair with Nonu in the midfield. A Nonu/Emery midfield has by far the most potential (and they would complement each other very well), but Emery is still very inexperienced (he is still only 19!). Treeby and Burleigh are the other options in the midfield - both can do solid jobs, but do have a few weaknesses (Treeby - distribution, Burleigh - physicality, defense).
Not sure what they should do in terms of a wing option to cover O'Donnell. I think they need a big powerful option in case Gear gets injured - Nemani and Lam would be two good options but are involved with the Chiefs WTG and the Sevens squad respectively (So we probably can't steal them.... not sure what the story is here). I wouldn't mind it if we got Kurt Baker back, but (a) he is with the Crusaders WTG, and (b) Joseph doesn't seem to like him very much....