While over the years there have been many advancements to the attribute modifiers for armor and tools that make each one unique such as attack damage, armor worth, cool down, etc. I feel there are some limitations. Here is some of my suggestions that would make for a better experience.
1. Custom MAX Durability tag for tools and armor (entity-wise)... yes if you can have a damaged leather armor, that's nearly destroyed but what if it's a custom named like Training Cap, it'd be nice to know it's actually at full durability and let's say it has a durability of 12 (at full capacity), then it'll be full and quickly go down. It's kinda of hard, especially, with so many maps using resource packs, to tell how long something lasts. This suggestion is #1 on this list because it goes a lot with my other suggestions and wouldn't really be possible if this isn't implemented.
2. Ability to mend and enchant tools/armor with custom attribute modifiers and tags where it'll retain those set attribute modifiers and tags. Let's say I have a stone sword that has a custom attack damage of 20, now let's say I'd like to enchant that item, while you can do this already, it loses it's custom tags and attribute modifiers and would return to the default for a stone sword of 5. Same goes for mending armor/tools using an anvil. The attack damage should update if needed to be for certain enchantments like Sharpness. This would add many survival-like aspects to custom maps. And with the suggestion from #1, it'd add a set max for the item's durability.
3. Alternate textures for the same item using something like META-DATA. Let's say I want to have 200+ different types of weapons, should I really be limited to 25 or so textures? Do all bows really have to look alike? this one isn't so important in my opinion but if done right it would be like adding new tools and armor into Vanilla Minecraft without mods. There could be a .json format to instruct Java/Minecraft what textures need to be used.
If all 3 suggestions are implemented, just think of the range of creativity that could be implemented into custom maps of all genres... Would go really well with 1.13 seeing as they plan to add custom crafting recipes.
What do you think?