This is kinda two requests in one, but both are somewhat similar in their goals.
Currently, you get the option of adding Starter Content or not; and you can add stuff like the Water Planes or the Kite Demo Collection to an existing project.
However, the former is an all-or-nothing deal, and the latter is currently only available from the launcher; not from within the editor where the project is currently open.
So, first request: Show all packs from the Learn tab and the Marketplace that can be added in the “Add Content to project” window. Even though I got the Water Planes for example, it only shows the Starter and Mobile Starter Content.
And considering the size of some packages (again, lets take the Kite Demo Collection as example), one might not want to add the whole thing just to use a tree or a texture.
Thus, second request: Allow us to select items to import, rather than pulling everything in. This also relates to “Migrate”, which currently uses the open project as source for the export - but I cannot simply go and say “Migrate from ThisOtherProject into my current project” (at least it feels a bit odd to me that in those cases, as the intended way seems to be “Switch to ThisOtherProject”, “Migrate into MyCurrentProject”, “Switch back to MyCurrentProject” if we could just skip 2 of the 3 steps by inverting the direction).