I hope the NFL never adopts the 4th & 12 to replace onside kicks.

I think it needs to be harder. Maybe a 4th and 20. But it’s the play that should replace the kickoff. That’s how the game starts, that’s how they line up after scores and to start the 2nd half. Teams can run it anyway they want. Punt, lineup to attempt to convert, etc. NFL kick offs stink and on sides kicks aren’t much better. This new wrinkle would be a welcome addition to put some life back in the “kick off” plays.
Yeah it would need to be harder. Gaining 12 yards in the modern game with the rules overwhelmingly in the offenses favor isn't that difficult. No lead is safe as it is and we see these crazy comebacks every week. I can't imagine what it would be like if teams could play make it take it. Teams with terrible defenses could go for it often just trying to get as many possessions as possible.

Something needs to be done because the onside kick is broken. 4th and 15 or 4th and 20 would be cool. Or 4th and 12 but the penalty for not converting is severe.