Universal trader
You can craft an universal trader out of any single item or block by placing them in a U shape (like cauldrons) but with the center filled as well. It will work with anything but the efficiency will vary from 1:10 for the cappiest material to 1:2 from diamond blocks or such.
The trader can receive items and will convert them into a milky rainbow sparling liquid called "pix" according to it's efficiency. The trader will be compatible with the NEI "?" button or any item you set in the second slot. It will then act like an automatic crafting table instead it will use pix instead of material to craft items (again according to his own efficiency). Allowing you to trade everything.
The Utrader will remember the last 2-3 trades you did in it and will improve the efficiency (up to double) if you keep using the same trades in the same trader. This will favor players who build insteresting production chains. Specially if they need import something as well. But it will also create the need for a bigger machines with lots of pipes and pumps to distribute correctly the pix between many specialised Utraders.
You: Oh Iluvalar, that thing already existed. It was called a transmutation tablet from EE. And It was a mess because players were constantly finding new way to abuse of it !
Me: Don't worry, I have a solution for that. At the start and end of the game, and maybe via a button in the trader GUI. The mod will send an HTTP request to a remote server. Making a repport about what you bought, what you sold and what mod pack you're in. The server would run a simplified economical model and return you the prices of each items¹.
This way, if you found an abuse and start bying blaze rods in mass and sell blaze powders in mass, the prices would adapt to your new trick. Blaze rods would become more expensive and blaze powder cheaper. Until you find a new equilibrium where you can't abuse of the bargain anymore. Those imbalance instead of being the desmise of the mod would just be a funny aspect of it.
Why it would be so great
This trader would allow us to break loose from the narrow path we all are actually forced to go to find our ressources. You don't want to mine iron ? You prefer to build a big plot of carrots this time ? Go for it ! You prefer to make yourself a bow and go hunting ? Go ! You want to explore the world and find some elusive magic plants ? Yeah go ! You want to startup a dirt factory ? Hell you gonna have to be good at it, but you can if you really really want !
We'll also be able to use the ressources around our base much more efficiently. I live in a swamp ? I can gather extra clay and sell my excess slime balls. I live in a jungle ? I have woods and cocoa bean for everyone ! I live in a desert ? I have cactus, sand and blaze powder to gather. The other way around, nobody would have to run around like an idiot during a 3 day walk to find a green dye, a single slime ball or a cookie because they are unlucky enough to have no relevant biome around their base. Unless you enjoy running around all the places in search of rare stuff... In which case you'd never had to stop exploring again
!
Everybody will do stuff that they like in the mods and won't be forced to do all the tedious other bit every games. We'd go much more creative and I'm convinced it would be great.
¹ As far as my knowledge of PHP and webhosting goes, it wouldn't be more expensive to maintain then a single page of forum everyday per player. I think it's totally manageable.