I can easily see two Aelves faction being Light Aelves and Shadow Aelves
Shadow Aelves could easily be Cthulu/marine Elves
Also there is possibility that they will release kits for both for example a box of infranty that you can assemble as Shadow/Light Aelves, Shadow Aelves could get Malekith/Malerion + Morathi, Light Aelves will get Tyrion/Teclis (as one model or two)
quite easy move :
- GW will sell minis that could work for both factions add some units they still have for both armies and here you go
Shadow Cthulu - Malerion, Morathi, Mistweaver,repacked hero new elite infrantry, new elite cavalery, monster and repacked 1-2 Corsairs/Bleakswords/Witch Elves
Light Elves - Tyrion, Teclis, some kind of hero maybe even Mistweaver,repacked hero, new elite infrantry, new elite cavalery, monster and repacked 1-2 of WhiteLions/Pheonix/Pheonix Guards/Eternal Guards/Skycutters
that would be 2-4 big kits (Malerion, Morathi, Teclis, Tyrion), new infranty, new cavalery, new monster or ship or whatever, repacked boxes, 2 BT (or even one big BT like Nagash with 2 faction and possibility to field them as one) .