This is a bit of a long shot, but hear me out

For starters, I don't even know if this is possible to code into a mod.

But, how about in the new 1.14.4 Mcreator there is a feature for custom villager professions and trades?

They could work something like this:

New Villager Profession 

The things you would need to add to make this valid is

 - A texture for the villager's clothes/costume thingy. The base biome skin would stay the same depending on the biome, but the clothes would change depending on profession, much like vanilla 1.14.4

 - A workstation block. (The block is chosen from pre-created blocks (custom or vanilla)). This block doesn't have to have a gui, it can be purely decoration. But a villager without a profession or with your custom profession would treat it like a workstation, restocking their trades and generally hanging about. It would also need a 'working' sound. When a villager is at their station a sound usually plays. A librarian is a page turning noise and a weaponsmith is a hammer clang. The sound can be chosen from already made sounds or a custom imported OGG file.

 - Trades. There has to be at least ONE trade per level, but there can be as many as you want in one. E.g. 3 trades in Novice, 4 in apprentice, etc. There cannot be zero trades in a level.

 - Hero of the village.  There needs to be specific item/s that can be thrown at the player as a gift when they have this effect.

Zombie villager:

There needs to be another texture for the clothes/costume for the zombie version of the profession. All things apply to this such as the trades being reduced after being cured, the same trades staying if it wasn't naturally spawned and other things. Here's a link for you with more detail --> LINK

New Villager Trade

 - What villager profession? This could be any vanilla profession, custom profession or a wandering trader.

 - What level? So this means what level is the trade unlocked at. (level 1 is Novice, level 2 is Apprentice, level 3 is Journeyman, level 4 is Expert, and level 5 is Master). This would only apply to normal villagers and not wandering traders, as a trader doesn't have levels. They work slightly different. I will explain later how this might work.

 - The sell item. What is the villager selling? This can be in any amount (e.g. two iron ingots). If it is a tool or armor piece there could be an option for random enchantments (good ones, so-so ones, or useless ones).

 - The sell item/s. There doesn't have to be emeralds involved in this, but if there is there should be an option for an average price, because vanilla villagers have random emerald prices. (e.g. 3 - 6 emeralds or if your item is more precious 34 - 53 emeralds) . The number of emeralds can rise and fall due to supply/demand and popularity within the bounds of the given numbers. There can be an additional item, like how a librarian asks for emeralds and a book for an enchanted book, or a fisherman asks for emeralds and some raw fish for some cooked fish. But if you don't want emeralds in the trade you can just say two different items.

 - How many time can this trade be used until the villager needs to restock that trade? The average trades until disabled number is 5 or 6.

 - xp received. Pretty self-explanatory. How much experience do you get from that trade.

 - Hero of the village. All prices would have a discount for a player with the 'Hero of the village' buff.

Wandering Trader trades

They work a bit like villager trades but not quite. They are split into categories such as: 1 emerald trades, 2 emerald trades, so on and so forth until a 5 emerald trades. The trader will pick 5 random trades from all of the categories (regardless of price). They will then pick 1 more trade from a separate list, usually being more expensive, as their final trade.

Again there is a limit to how many times you can buy that item until it locks out forever.

More info here

Well, that's my idea. This will probably never happen in 2019.5, but maybe in future versions this could be possible. Please consider this!