For New Zealand, this is how I'd go:
Props (6): De Groot, Lomax, Tu'ungafasi, Williams, Moody, Ross ------------ toughest omission: Laulala
Hookers (3): Taukeiaho, Coles, Taylor ----------- toughest omission: Aumua
Locks (4): Whitelock, Retallick, S. Barrett, Vaa'i ----------- toughest omission: Tuipolutu
Loosies (6): Savea, Blackadder, Sotutu, Papali'i, Jacobson, Cane (S Barrett) ------------ toughest omission: Frizzell
Halfbacks (3): Smith, Roigard, Christie ------------ toughest omission: Fakatava / Weber / Perenara
First Fives (3): Mo'unga, B Barrett, McKenzie ------------ toughest omission: Perofeta
Midfeilders (4): J Barrett, ALB, Ioane, Goodhue (Faianganuku) ------------ toughest omission: Havili
Outside Backs (4): Jordan, Telea, Stevenson, Faianganuku (Ioane), (Barrett x2), (McKenzie) ------------ toughest omission: Clarke
My starting 23 for the WC, based on current knowledge, would be:
1. De Groot
2. Taukei'aho
3. Lomax
4. Retallick
5. Whitelock (Captain with Savea as Vice)
6. Blackadder
7. Savea (Assuming Blackadder is back to his usual self before WC, this would be the best loose trio IMO. Sotutu's a great player in great form, Savea is Savea... unlucky Cane)
8. Sotutu
9. Smith
10. Mo'unga
11. Telea (have to pick him on form. Moved to 11 where he is just as good in order to make room for Jordan/Stevenson)
12. J Barrett
13. ALB (Better 13 than Ioane for me, assuming he maintains form. Great decision maker, great skills, great physicality. Ioane lacks passing and decision-making)
14. Jordan
15. Stevenson (If Jordan gets back to his best before the WC then I'd swap Stevenson and Jordan. For now, Stevenson is our best FB)
16. Coles (people underestimate how good Coles still is. He's still dynamic. Valuable gamesmanship too)
17. Williams
18. Tu'ungafasi
19. Barrett (If Cane starts then I like Barrett at 6 with Savea at 8. In my team, though, Barrett makes a perfect impact sub at lock or 6)
20. Papali'i
21. Roigard (undeniable form, and his style would be perfect as an impact sub)
22. McKenzie (edges Barrett based on current form.... but I could go either way, really)
23. Ioane (edges Faianganuku as center/wing cover, again could go either way)
Those are my thoughts. I know Foster's will be different.... especially re Cane and probably the centers and back 3.