That seems to be the likely scenario where Vexen would be headed. As far goes as data and experimentation I say that San Fransokyo is written out for Vexen as Data Riku/Repliku is the Main Seeker/Org. Member for this world
(unless if Vexen shows up at the very end of the world, but that's unlikely). Wreck-It Ralph also looks to be out of the question as it might be the new world hinted for Union X, same goes for Tron I don't think we might go back to this world sadly.
The only Disney world I assume would fit Vexen's experimentation is Lilo & Stitch. Apparently, both Dr. Jumba Jookiba and Dr. Hamsterviel creating genetic illegal experiments do parallel Even, Terra-Xehanort and the rest of Ansem the Wise's apprentices making illegal experiments on the human heart and the darkness from within it which lead to the creation of the Heartless. Of course, I'm not sure about the data part cause the Lilo & Stitch didn't delve too much on data it mostly dealt with experimentation. So that's 50/50
I would be surprised if it's for a different Disney world (e.g. Agrabah, Oz the Great and Powerful, Atlantis) or an original world that he ends up that might be unrelated to data and experimentation, but that's just me.
As far goes as Saix, who knows, I know I shouldn't get ahead of myself but it doesn't seem he will be the Seeker/Org. Member in a Disney world that SDG will encounter as it seems he might be going after Kairi and Lea, mainly the latter as he would serve as a foil to the both of them. As we got two more Disney worlds left belonging in the main set it seems that the following words will be this:
- Xigbar: Olympus
- Ansem and Xemnas: Twilight Town
- Young Xehanort: Toy Box
- Vanitas: Monstropolis
- Demyx: Princess World (Princess and the Frog)
- Marluxia: Kingdom of Corona
- Larxene: Arendelle
- Vexen: Unknown Disney world in the third set (Lilo & Stitch, Oz, Agrabah or Atlantis)
- Luxord: The Caribbean
- Data Riku/Repliku: San Fransokyo
- Master Xehanort: Keyblade Graveyard/Kingdom Hearts
- Saix: ???