Wait, so the Eion will be playable some day? Cool!
Why not just have the Athens AND Eion playable for the UNSC? After all, they built it. The Rebels/insurrections just stole whatever they could and used it according to the lore. Heck they used warthogs and chopped the back off, and replaced it with a new rig, trying to turn it into an APC during one of John' early "training" missions in the early days.