[set fair (OP)]

What are the difficulties? Could it be done?

[alancalverd]

Difficulties:
1. Specifiying the requirements
2. Determining how to meet the specification
3. Producing and testing the prototype
4. Patenting all your hard work
5. Proving that your dreadful invasive alien monster bean (not my words) will not infect and interbreed with any precious native species.   
6. Ensuring that the crop is sterile and cannot be replicated by sowing, cutting or grafting (or you won't make any money out of it!)
7. Proving that it won't cause cancer/asthma/ADHD/PTSD/hair loss/hair growth/hypochondria or any other fashionable ailment
8. Obtaining adequate insurance against diseases not currently fashionable
9. Growing it - which will open a whole new can of worms/beetles/birds/bees/environmental impact....
10. Making it palatable
11. Selling your Frankenburger in a hostile and competitive market.

Selectively-bred high yield wheat and rice have been around for 50 years, and are still controversial. Once you start playing God with bean genes you will face excommunication and the wrath of the Daily Mail.

And who wants to get all their dietary needs from one bean? It's great in an emergency, but the profitable market wants a more complex flavor and texture in its enchilada.

[Bored chemist]

It's easier to put your beans on toast.
Neither wheat nor haricot beans provide all the amino acids we need, but, between them they cover our requirements.
Or, at least, so I'm told; I never checked.