I have thought about this question, @poisonedantidote, and have considered two approaches as interesting thought experiments. I don’t know if anyone has tried these ideas; I’m not in a position to implement them, so I haven’t done any research.
Actually, my focus was never on “the best” or “the perfect” method but rather just “a good or effective” approach. Perfection is a pretty useless concept to me except in a theoretical context.
I’ve thought it would be interesting, first, to design a course to simulate the way children learn their own language—that is, not just with immersion but with a speeded-up recapitulation of infant experiences, focusing on eating and sleeping, comfort, cuddling, warmth, and love. I don’t know how that would work in practice for adults, but I’ll bet something effective and practical could be designed without inviting crude humor and inappropriate behavior.
Second, I’ve wondered what would happen if someone chose to teach a language—say, English—by starting with the way English-speakers speak their language. Let’s say your native language is Greek and you want to learn American English. Suppose I started not by teaching you any English but by teaching you to speak your own language, Greek, like an American? You’d learn an American accent, you’d learn typically American mistakes in Greek usage, you’d learn typical American errors with Greek grammar—errors that most likely reflect the sound and structure of English rather than Greek. Maybe you’d even practice writing Greek words using the Roman (English) alphabet. Then once you got used to sounding and thinking more like a Greek-speaking American than a Greek-speaking Greek, we’d start replacing Greek vocabulary with English words and adjusting the grammatical structures.
@john65pennington, I don’t think it’s universally agreed that English is the hardest language to learn, and I also wish English-speakers would quit boasting about the oddities of English. Relative difficulty might depend at least a little bit on what native language you’re starting from. A Finnish woman whose English was excellent told me that she had a much harder time learning German.