Gap Tool and Live Distribute like InDesign has, with improvements
InDesign has Gap Tool — https://helpx.adobe.com/indesign/using/aligning-distributing-objects.html#align_objects_using_the_gap_tool
Also it has Live Distribute —helpx.adobe.com/indesign/using/aligning-distributing-objects.html#distributeobjectsusinglivedistribute
These allow us to control a gap’s size between chosen pair of elements (quoted):
—Drag to move the gap and resize all objects aligned along the gap.
— Shift-drag to move the gap between only the two nearest objects.
— Ctrl-drag (Windows) or Command-drag (Mac OS) to resize the gap instead of moving it. Adding the Shift key resizes the gap between only the two nearest objects.
— Alt-drag (Windows) or Option-drag (Mac OS) to move the gap and objects in the same direction. Adding the Shift key moves only the two nearest objects.
— Ctrl+Alt-drag (Windows) or Command+Option-drag (Mac OS) to resize the gap and move the objects. Adding the Shift key to resize the gap and move only the two nearest objects.
— Start dragging a selection handle and hold down the Spacebar while dragging. Continue dragging to change the spacing between the objects.
Illustrator needs these options as well.
However, we need more than it has.
We need a way to numerically set these values too, not only interactively on canvas. The only thing we have now from the box is Distribute Spacing, and it’s rather limited.
We need a way to specify the final dimension of the selection or the final position of the chosen edge (similar to the way this script allows: https://sysys.blog.shinobi.jp/Entry/96/)
We need a way to choose an axis or the path (curves or straight) these gaps follow.
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Now imagine a key object being able to control Live Distribute... We click the middle one to stay in place, resize the right edge with Space held, and the left edge gets shifted simultaneously, so that the middle one stays in place, while other objects move... to get the row shown at the bottom from the one at the top.
Ai underutilizes the key object paradigm greatly :(