Describe the feature you have in mind.

Basically, you take the copper backtank as the base, and add a new blocked called "Tether". Like the normal backtank, it needs rotational power in the top to draw in air, which it then holds in an internal storage. But, unlike the backtank, you use the air by wearing either the backtank or jetpack within range. This will drain air from the tether, but will fill up your worn supply.

Does this solve a particular problem?

It can be annoying to constantly take off the jetpack to recharge, especially if you're mid build, so I made a datapack that does as above. Because the tether still needs a rotational source I believe this addition still matches the feel of the Create mod and this addon.