Ok, here is what I think is missing in dive industry: a tank valve and a first stage that allow to swap tanks under water. That is, a specifically designed first stage and the valve where under water you can take the first stage off and put it on some other tank without surfacing.
I guess how many times would a diver need this feature ?
At least, in my short dive experience, with rental and my own reg set, I never had the need to do this.
If a feature is to be designed and implemented, is because there is enormous need and/or money behind.