God forbid we come up with the gangster version of AfterMASH. The universe would fold over on itself or something.

The more I think of this, a prequel set in the mid-1980s to just before The Sopranos began (circa 1997-98) will not work. Either you have older actors trying to play younger versions of themselves, or different actors in the same role (notably a younger Tony). It's just asking to be mocked, and would be a parody of a great show.

The only way a prequel has a shot is going about 30-40 years before The Sopranos original run (probably 1961 to 1979). And like the flashback sequences, it's going to require an entirely new cast, because the characters are going to be starkly different. You'd have the rise of Johnny Boy and Junior, obviously younger versions of Paulie and Hesh, but I doubt you'd have a younger version of Big Pussy (on account that he didn't rise up until the Unrest of '82). You'd probably have an adolescent Tony and his crew of Jackie, Silvio and Ralphie, and maybe Tony Blundetto. You might even work in the Feech robbery.

But unless you have a writing staff dedicated to crafting workable, captivating story lines without drifting into self-parody, even this prequel has a high probability of failing. Couple that with higher production costs of a period series, you'd be hard pressed to find HBO willing to green light it.