It would be a good to have autocomplete in form field text type. I know this can be done via javascript but I think it would better if this were inbuilt to automatically use the existing rows of the model to which the field belongs to show autocomplete options. By default this could be false and it can be an option to set true when a field requires it.

What should it autocomplete? The text form field just contains text and has no direct link to any of the models? Or am I missing the brilliance of this feature?

Previously added entries--to suggest those so that user can click the down button in keyboard and hit enter than typing entire fields. This is useful where some fields are repetitive i.e. like if you use the same name again and again for several records. So, it could be an option in form field definition to allow this autocompletion to be turned on.

An autocomplete features could be useful as a second option to display a relation type. Right now, the simple relation type is display as a dropdown handled by a jquery select library. It is very nice> However, when you start to handle some serious amount of data, this jquery library is exploding> I have a user profile linked to a company trough a relation. In the user profil form (backend), we can select his company. The problem is that i have around 40 000 companies, and the select dropdown does not show up every time, sometimes i need to wait 50 seconds. Hence i think in that use-case, an autocomplete feature has a real concrete purpose.

From the video, I see that an autocomplete field is being used in the Builder plugin to complete model names. Since the source code is available on github, I suppose I can dig thro it to find the solution to this.

To implement autocomplete for backend form field you can use a plugin: Relation Dynamic Dropdown Form Widget Plugin.