Holograms at the moment are fairly useless. They can be used to make statues of players that can be any size, and that's about it. 

This suggestion will cover a major overhaul for holograms I thought up.

SECTION 1: PLACING

After crafting and placing a hologram, it will be bound to your UUID (similar to a TARDIS). This will prevent people breaking or changing the hologram (more on this later). 

Placing can be used with the current format, where renaming the hologram to username or username:1 would place the holograms as a regular hologram or a solid hologram respectively.

SECTION 2: CHANGING HOLOGRAMS

Holograms will now be changed via a UI, obtained by sonicing the hologram block. The GUI will have 3 buttons along the left, which will bring up different tabs. Display, Permitted Editors, and Break. 

DISPLAY: This will be the tab that automatically opens when you sonic the hologram. In here, there will be a few buttons and some text inputs.

• The first text field will be for usernames or image links (direct imgur links, discord image links, basically anything that ends in .png). The way links to skin files would work is similar to More Player Models. The determining factor for whether the hologram sees the text as a name or a link would be http:// or https:// at the front.
• The next line would contain 2 buttons and a small text field. The first button is Solid, the second Small Arms. These are both carry overs from the nbt (the :1 and the nbt {small_arms:}. The small text field would be labelled "Scale", and would accept all numbers that render properly in the hologram (maximum of 50)
• The next few lines all have specific text fields. These will allow you to display armour and items in the head, chest, legs, feet, mainhand and offhand slots, exactly like an armour stand. Unlike armour stands, however, the armour and items can be set to render holographic or solid, similar to the skins on holograms. This would be toggled by a button at the end of the text field box. Also unlike armour stands is that you don't require the item in order to put it on the hologram, just the item ID, as it will be display only.

PERMITTED EDITORS: As the name suggests, this tab would store all people able to open the UI to the hologram. Only the owner can modify this tab, however all with access can view. It would look similar to the TARDIS Waypoint GUI, with the name input, and a button to both save and delete names from the UI. Below the buttons to add and remove editors would be two buttons which only one of can be selected at any time. These would allow for everyone to be able to edit, or just the people on the list.

BREAK: This button will break the hologram similarly to breaking a shulker box, dropping the item with all NBT and settings saved, for moving the block without having to redo all the settings and such. That is all it would be used for, and is only usable by the hologram's owner. Breaking the hologram outside of the GUI would just drop a plain hologram item, and is also only able to be done by the hologram's owner.

SECTION 3: WHY THIS SHOULD BE IMPLEMENTED and CONCEPT IMAGES

This overhaul would allow for more uses of holograms for building and maps. It would also allow for more customisation, with any skin in existence being able to be a hologram, instead of just currently used player skins. It could also allow for HD skins to be displayed, as well as more ways to display items to show off builds more.

Extremely rough idea for the Display tab

Extremely rough idea for the Editors tab

If you have any more ideas, or just feedback on the concept in general, please leave them in the replies. I honestly hope this feature can make it to a future build of the mod (maybe 48, maybe further away in the future).