Will depend on how many minutes they get against Fiji and how the squad as a whole pulls up, but I'd like to see Hooper, Tizzano and Daugunu considered to ensure they get a crack against the Lions. As the squad stands it probably only makes sense to include one of Hooper or Tizzano though, in which case it's probably Hooper as I think he's the least likely to feature in the tests. Daugunu also has the advantage of being eligible for the First Nations/Pasifika game as well. They will need a few more guys than 23 in the squad though, so it still makes sense to call them in (possibly forcing some of the other out of the 23 too).
With that in mind I'd probably fill the gaps with Asiata, Hooper, Gordon (if fit), and Lynagh (if fit). I'd then have either Teddy Wilson or Kalani Thomas covering Gordon and ideally O'Connor covering Lynagh, but Burey or Stewart could be good options as a utility instead. Tupou, Swain, Faessler, CdC and Pietsch all make sense as coverage options they could bring in to round out the squad.