My suggestion:

From:
If you wouldn't exist, you should be invented.
To:
If you wouldn't exist, you would have to be invented.

@Eldad: I don't know, the meaning I'm trying to give is pretty much like "what would the world be without you?", though in a very roundabout way to say it : )
the translation is almost literal from Spanish, but maybe "you should be invented" doesn't sound like it should.

I understood the Spanish; I thought the current wording would be OK in English and would also render the Spanish properly. However, as always, I'm open for corrections and suggestions for change.

As the sentence has been disowned, I'm adopting it :)

Yes, "If you didn't exist" etc.
I accept and would make the change; Marcelo, I believe it matches the Spanish, what say you?

"If you didn't exist, you would have been invented."
sounds great to me.
It's not very literal, but it says exactly the same, and more accurately

Changed.
I removed the @NNC tag.

Why? :)

Thank you very much for the explanation and for the useful links.