TL;DR
To easily, visually recognize that the game (especially in multiplayer, with lag) accepted your copy/paste, temporarily highlight the entity for which an action was performed.What ?
The entities should highlight in a blatantly obvious manner when copying/pasting.It would be ideal if it was an optional function, since some people may not like it.
Why ?
This functionality would visually show that the game accepted a command of copy/paste on an entity.As it stands, you can't quite easily (easily is the key word) know that a copy was successful, and you absolutely cannot know that all pastes were successful without alt-info-capable entities.
With alt-info enabled, you can glance and see that pastes were accepted on a handful of certain entities, but most others (chests, trains, cargo wagons, inserters, combinators, train stops, pumps, etc) require that you hover over every single one and occasionally open a new dialog window (changing 2750 to 2755 isn't apparent on a hover) to confirm that your intended action was accepted, effectively forcing you to perform an action at least twice. I often run both directions when pasting and swipe my mouse around as well, because that's faster and more reliable than running once and then checking every single chest and cargo wagon, but it's still an undesirable action to have to perform regularly.
This visual confirmation becomes even more of a need when running quickly, or simply being on a multiplayer server when running and pasting at a slow speed, but you have to individually check every chest/wagon to be sure that it was setup correctly.
If it's able to be shown in a multiplayer environment when others perform actions, it would be a great addition to see what they're doing without having to ask, since you can't see their cursor or keystrokes.
Here are some quick examples of an assembler, chest, and cargo wagon. I'm sure that with all the fancy stuff that you all are capable of doing, that there's a better way than making duplicate sprites with colors added, which could also allow for one color for copy and another for paste.