Chaostone is an ingot of glowing blue metal, that is found in the end abundantly in basestones, which are rare, smallish rough ovoids near the bottom of the end's outer islands, 800m from the inner edge of the out end islands, made of endstone, obsidian, and chaostone ore, which resembles crying obsidian that glows, and produced many ingots of chaostone, without the need the smelt. Chaostone can be crafted into a portal, with 8 chaostone ingots and an eye of ender in the middle. They can also be used to make a portal setting tool, with a stick at the bottom, ingot at the top, and eye of ender in the middle. Portals can be set by using the tool on one, and then going to another to use the tool on the other portal, linking them. Linked portals look like the portal to the outer end islands. Portals cannot be linked more than 200 blocks.
This would be very useful. It could use a few more uses, but the ability to set teleporters is already game-changing so it would be far from redundant. I especially like it can only be found in the far end.
Teleporters would be an interesting use for the chaostone, and like fishg said, teleporter would be game-changing and I also prefer if it can only be found far away from the end gateway portal. Personally, I'd say that the chaostone should be rare, spawning in a veins of 3-6 and only drop 1 ore which you must smelt. Therefore, you must travel very far to make the best out of it. The basestone should only spawn 800 blocks away from the end gateway portal and the basestones would be disconnected from the many island, forcing you to build a bridge over to it. The actual basestones is reasonably common, but finding one with any chaostone isn't easy.
With that, it seems perfectly balanced, especially since these things aren't easy to come by. While I think the limit should be smaller, we can never know for sure what is the most balanced until it's in the game.