It would be greatly appreciated if the ability to alter the Attack/AC bonus' of armor and weapons could be added via the character sheets "Customize item" feature, like you can it's weight or name. This way, instead of making us take the time to Homebrew, say, Mithril plate +1, +2, and +3, The DM can just say "Your Mithril Plate is now +1" and we can just change it from +0 to +1 in the Sheet. Keep in mind, mithril Plate is just a example, It could be anything that doesn't already have a +1,2,3 version. Plus, this way, instead of making the player remove the "Quarterstaff" and replace it with "Quarterstaff +1", the Monk can just change the +0 to +1 on the Quarterstaff they already have in their inventory.

Edit: It seems you can already do this with Weapons (Be able to modify their "To hit" and "Damage" Bonus) so why can't we already modify armors AC bonus?

I think the main reason they do not do it is because Wizards does not want to implement anything that is not RAW, and expanding too much support for homebrew to include things outside of RAW just does not seem very financially sensible.

What I can see happening is that once they implement the system to support moonblades, Vestiges of Divergence, piety artifacts, hoard items, etc., you will just need to homebrew the item properly once, and then select the relevant options and upgrades from the character sheet afterwards. You will still have to go through homebrew, but once you homebrew an item once, you do not have to keep going back to the homebrew tools to make further adjustments.