If I get this right, you think the "mail receiver" works fine: thus what do you want to rewrite?

+1 this would be useful.

It could be implemented via a new rich text editor for the Knowledge Base system.

I don't understand how a "release" is part of an inventory asset?

Are you referring to a plugin?

Disclaimer: I like the fact that additional features for GLPI are possible via plugins: this makes the GLPI core less bloated.

Why integrate a plugin into the GLPI Core? If the plugin does the job there is no need to make it part of GLPI.
If you think the plugin's features should be integrated into GLPI's core: please explain why it should be, which advantages it would brings, why the plugin-solution is not working (or badly).

What is an "unused document"? A document object not linked to anything?

In my case, I use GLPI to store documents such as server manuals and they are not linked to any asset (because the task of linking it to all our assets would be tedious).

A member of an group which is assigned to an Asset can't choose that asset on creating tickets.

This looks like a bug or a configuration issue.

To make group's assets displayed on ticket creation, you have to configure the profile accordingly:

In the given profile configuration (eg. post-only) > "Assistance" tab > "Association" section > Check "See hardware of my groups".

Aren't they already created on LDAP sync?

I think such question should go to plugin's bug/issue tracker.

It seems v0.85 compliance is already done but no sign of a v0.90 one.

I'm +1 on this one but I'm also aware of the big changes involved to offer such search system (available for every "search" form of GLPI).

Maybe an "advanced" single input field with some search oriented Domain specific language (DSL) but this would not be user-friendly.

I think such request should go to plugin's bug/issue tracker.

(And it has now been ported to v0.90)