If you have once worked with OTRS/Znuny (open source) this feature is really missing in daily use, and for reducing/eliminating the use of an external mail client/Outlook. You should be able to comunicate with a 3rd party, without adding this party to the conversation with the requestor and vice-versa.

Take this simple task: the requestor asks for a device/software, and I need to contact the supplier for an offer.

OTRS resolves this in a very simple, very transparent way:
You always see to whom you forward a ticket (and which attachments are added), and responses from 3rd parties arrive as a private follow-up and thus do not generate notifications to the requestor.

Also other competitors (Jira,...) miss this feature so it would be a selling point for GLPI.

Nowadays user assignment is essential even for a basic license management.

That would be really a useful functionality for GLPI! We know who is assigned to an asset, but for now the only way to show these assets to the user are dropdown fields in ticket forms. An overview "My Assets" (just with basic info: name, manufacturer, model,...) e.g. with the possibility to open a ticket starting from the asset, or to create a ticket that contains all these assets to confirm that you own them - some company needs to do this inventory yearly. 
Configuration: per entity, it should be possible to configure the asset types that are displayed in the simplified interface.