Trading with a villager enough times should let you alter it's trades

Trying to get the specific trades is boring, random and not at all engaging. People are going to try to get their desired trades even if they're random, so why not make it more fun and rewarding? If you use a specific trade enough times you should get the option to swap the trade. You shouldn't be able to change trades into different tiers, so a level 1 trade can't be replaced with a level 3 trade,for example. Also, for example, you shouldn't be able to give a librarian the trades of a blacksmith. A requirement for swapping a trade should be that you need to give the villager the item that you want it to trade. If you want to trade a mending book instead of a thorns book with a librarian, you need to already have a mending book at hand before swapping the trade. This would be a good way to not make it too OP. After the trade is replaced, you need to use that trade more times again before you can swap it out. Another suggestion that goes with this is to make certain books from librarians higher level trades than others. Mending is quite OP, so it makes sense if it's a higher level trade and maybe costs more on average than something like Smite 1.