Scale the space between objects
The ability to move objects in uniform space, similar to how the bounding box works when you select multiple objects and then scale them larger or smaller, if there was way that when you scaled the bounding box bigger the objects maintained their current size but moved farther from each other.
I notice there are times when I have objects on the page and I need to move them apart uniformly based on spacing or move them closer to each other.

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Recently Astute Graphics developed a plugin tool that does exactly this, called Inflate Deflate tool: https://docs.astutegraphics.com/colliderscribe/inflate-deflate-tool
It provides similar but different method, and won’t allow precise numeric operations.
The scripts mentioned below and the old-reliable Scale + Transform Each combo still work as an alternative.But yeah, an ability to just grab the handle of the bounding box with Space held and drag it to alter the space, like InDesign does, would be great...
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Brendan Cummings commented
It would be nice to have a tool or other way to move objects relative to one another. I think the best example of what I'm suggesting is what happens when you have two anchor points selected and use the scale tool. The anchor points move towards or away from another. Unfortunately, you can't do this with the objects as it would scale object dimensions as well, causing them to stretch.
This would be very helpful for when I have objects in line with one another and need to move them farther apart or closer together at an angle that is not divisible by 45degrees.
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There are two scripts you can use to workaround this:
https://sysys.blog.shinobi.jp/Entry/96/
https://sysys.blog.shinobi.jp/Entry/132/
Both are free, both are for Illustrator (don’t trust the type one has). Don't be afraid of the Japanese, they are both translated into English.
There are minor flaws (they always 'use preview bounds', which makes it less useful for stroked objects), but they can help you in many cases. -
Thomas Hale commented
This has become a feature ingrained in my muscle memory from using Adobe XD. I like that when I am trying to sanity check and see that I am indeed pixel perfect, I can hit "Alt" and see the amount of space between shapes/items.
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InDesign has this, and I really would like to have this in AI also:
http://dualpixel.com.br/labs/wp-content/uploads/2017/10/live-distribute2.gif