What I am suggesting should not be considered within too narrow of a scope. It is actually a large swath of MEP elements that would become possible with this one simple option available.

In this CADmep fitting, the tap connection point adapts to the size of the pipe that it is coming off of.
I'm fairly certain that Revit does not allow this at draw time with attaches to (or similar) fittings. Those that can read the main size also split it. I've looked far and wide. You can make a manually entered dimension that reads this, but this is a big deviation from the Revit philosophy. We need a way to read the size of the main for fittings like the one above.
But like I said, it's not just good for that.
Here's something else that could benefit from it

The new hanger in this image works much the same way. It's attaches to, it does not split the system, and it needs to know the size of the pipe that it came off of. The difference is that it is rotated and is not a junction for branchline.
Now I have another element type that would benefit. I will just widen my screenshot

The connector. If you take a look around at all of the AddIns being developed for Revit and MEP purposes, you will find a common trend when it comes to connectors. They are union element types that split the duct/pipe at specification governed intervals.
While that is more familiar to CAD philosophy and and more closely resembles reality, let's consider this: In most cases, all that the MEP trades need connectors in their models for is to determine cut lengths. You could just as easily establish that with elements that attach to the single, long system family in an array. But the system family would maintain that Revit-specific benefit of being flexible and parametric, because it's still one element (with an arrayed family on it) as opposed to several pieces that will throw errors when nullified or otherwise.
I've come very close to accomplishing this connector that I describe. Some methods of building the family are a little sketchy, but the only thing truly stone walling the process is the inability to read a main without totally splitting it.
The bottom line is that attaching to a system without splitting it and still reading the system size is a common need for us. You wouldn't be doing it as an addition for those "rare instances". It would be used all the time. It would be a core function. Please consider this. Thank you.