With extremely large inventories, finding just about anything in the masses of items becomes a chore. All too easily you will miss something. It's especially harsh in missiles, where the arrows and quarrels and rocks are completely jumbled together.

The item group filters are rather useful, but don't help a lot to discern things within individual item groups. If at all possible to implement, I don't think there are many better solutions than a filter that only shows items that contain a set of characters entered by the player in a prompt accessible in inventory-type screens that would allow item filters at this time. For instance, entering "arrow" would filter out all items in the inventory that do not contain "arrow" in their name, finally allowing players to browse their arrows undisturbed from quarrels and rocks. "slaying" would detect all slaying ammunition (and affixed weapons) and so on. It would also provide players with a quick method to check if they have a certain item without scrolling through the inventory much at all, like entering "danger" to check for scrolls of danger.

You could connect the prompt to the "Ctrl-f" key combination as a way of having ADOM pretend it's a browser.

EDIT: Another suggestion relating to the item filters (including the already established filter by item group) is to make them available for the Ctrl-p and Ctrl-d screens - the ones that let you mark items for picking up and dropping.