There's a reason this was never implemented.

And you probably don't see it because your current categories all have the same layout.

At the moment, when you combine different categories, the only thing that is essentially overwritten is the grid. Beyond that however, you must expect that when you click a given item, that item inherits its category's display settings. So if it's a regular article, the image is e.g. right after the title. If it's a video, the image is replaced by the video player right after the title (either by using different overrides or different content settings - e.g. the content and image could be completely hidden in that case).

If these 2 different entities inherited from a single custom-chosen category, you'd eventually view them with the same layout. And what happens if there's another K2 menu item pointing to a different set of categories, some of which are common to the first menu item? Can you see where this goes?

With the current setup (and contrary to Joomla articles' layout inheritance based on the menu item ID), the resulting URL and layout will ALWAYS be common.

Even so, K2 gives you the tools to perform a possibly different layout as you ask. You'd just have to create a different set of overrides. And then make a new K2 menu pointing to the categories you want and just assign that override set to them.

Yes, global settings is a better option and is something that will come with v4. Exactly as you have common settings now for the generic views, you'll also be able to configure default settings for categories and expanded details for tags, users etc. The point is to normalize the output so that category, tag etc. all share the same HTML and display options.

We'll get there :)

As for the specific example you mention, sure it makes sense, but how often would you change your grid settings? Since you have 500 menus, the simplest way to change the grid now is to run an SQL query to replace the old grid with the new one, en masse.

Surely after the summer for v4 as it will be J4 compatible as well. v3 is skipped... :)

What you could do in the meantime is create a simple K2 plugin that hooks on the "onK2BeforeSetQuery" event and modifies the query's limit to the new value you want & additionally resets the leading/primary/secondary/links group limits.

The plugin would require a menu selector (so you can choose where to apply this filter) and new grid settings in the form of leading count & columns, primary count & columns etc. These would be your XML fields for the plugin.

Adding the values from the new grid gets you the new query limit which you can enforce using the "onK2BeforeSetQuery" event. That event modifies the actual query (see github.com/getk2/k2/blob/master/components/com_k2/models/itemlist.php#L335). Use JRequest::setVar('limit', $limit); there to reset the query limit. It should work.

Finally use the "onK2BeforeViewDisplay" event to reset the leading/primary/secondary/links group limits.

Use the example K2 plugin as a basis for that (github.com/getk2/k2-example-plugin). It'll just be the XML file options (standard fields) plus the main plugin file hooking up code to the 2 events I previously mentioned.