I don't know if this has been talked about before, but i would love to be able to use some items in preparation for battles. Like imbuing my blade with naptha/blood iris outside of a fight, and having it's buff until the end of the next incoming battle, or at least to a certain number of hits, like let's say, 3 or 4 hits if they manage to hit the target, then having the buff go away. What i noticed is that, these items are very similar to Charge Weapon and Blue Flame Blade, but they don't have an additional effect like the abilities do (charge weapon dealing aoe damage when cast and blue flame being able to ignite enemies with crits); yet, those items end up using a turn just like the abilities do, but they don't have any other effect other than the on hit buff. I may not know a lot, but it sounds like a healthy change/addition to the game, along with letting people roleplay a bit more, if they're playing with a cautious champion or something like that.
For conjurer's concoction and other items that increase stats, which you imbibe, you could have the effect last a number of hours, but having it end when you finish a fight as well. with the added balance change of not being able to use more than one or two consumable item/stat buffs at a time, so as to not be able to use all items at once and having super high stats (although that wouldn't be a really viable strategy as all items would only last a single battle so doing it repeatedly with all items would be insanely expensive, not to mention also boring since you'd have to apply a bunch of items every time you finish a fight, so i imagine the mechanic would work only if you want the ropeplay bonus/a little buff for a fight).
It would also add a bit more of a roleplay element to those items, since you can prepare for battles on your own, instead of doing everything in the middle of a fight. As example, when going into the WC, it would make sense for the champion to know they would face boreal elves (which are weak to fire), so going prepared with naptha already applied for when a fight occurs would be a nice element of preparation, especially if a person is going through the dungeon for the first time, and does not know when a fight is about to break (also the characters there are strong, being able to dish out damage or a debuff in a round is more valuable than spending it buffing an item for 20-ish plus damage). I think an addition such as this would make for very interesting and healthy moments in the game for players, but then, i don't know much about D&D, like if naptha/blood iris items work in similar ways to other items in D&D campaigns (if they exist there), so writing the suggested addition above could deviate from the item's intented goal.