Thanks for mentioning LaunchKey (https://launchkey.com/). I am a co-founder and can confirm that we are doing something very similar but are only in the Authentication space. LaunchKey supports Private Keys stored on your device among other factors. We have iOS, Android, PHP, Ruby, Python and Javascript SDKs with a WordPress plugin and other integrations coming soon. Our system also allows session based or transactional authentication.

That is all good, but one of LaunchKey's biggest features is the Privacy. Each site/app you login to is provided a unique ID that cannot be traced or tracked among sites, ad networks, etc. If twitter opens their 2-factor login up for 3rd Party, it will surely be using this login data in other ways.

Check out LaunchKey (https://launchkey.com) and let me know if you have any questions.

p.s. Our app and system also lets you log out from your device.