I believe it would be nice to be able to modifly the way our weapons and shields look. Like applying skins (rather technical term, in a simple term you could call it "paintjobs") to them. Currently you can only modify armors, sadly.

For example, I have a very unique weapon,
Lionheart Honor Blade
5* Leviathan Blade with customized looks.
Its a work of art, compared to good old Leviathan Blade. I'd love to use it in all of my runs. But being a UV hoarder, all of my weapons have a good UV or two. I tried using Lionheart, but reached conclusion its not possible. UVs are a must for me.

So my suggestion is this:
Allow players to customize the looks of their weapons, shields and, possibly, other items, item use effects (like shields with different color, weapon swings of different color ect).

How to make money on this:
offer specialized, rare, unique skins for a limited selection of items, available only with special promotion energy purchases.

Now, there are several ways to serve these new skins (on example of a sword - Leviathan Blade):
1. Make new skin only available (applied) for a specific item. Like applying an accessory, this new skin is attached to the weapon. With it, weapon looks differently, saving its UVs, star rating ect (in case of Honor blades, only skins of items already in posession should become available to players).
2. Make new skins bound to a knight. This specific knight can then go and apply this skin to any weapon of this type. This changes the way the weapon looks, but its UVs, heat, star rating remain the same.
3. UV transfer. Take a weapon of a specific type (Leviathan blade), which contains UVs you want, take a weapon of same type (Lionheart honor blade) which looks the way you want it to look and combine them! This destroys the weapon with UVs (Levi), but all UVs are then transfered to the new item (Lionheart). In case of Lionheart Honor Blade it would make the item with good UVs permanently bound to player (cannot be traded).
4. Skin tranfer. Similar to UV transfer, but destroys the skinned item, updating your UVed item. The only difference is: Lionheart Honor Blade is something you can't upgrade or trade in any way, thus its permanently bound to you at a specific star level (not only 5* is available). Skin transfer would allow knights to trade their Levis with good UVs and great looks.

Or think of your own way to change the look of items! If I like your variant, I'll add them to my post, keeping author's name.