Why not say that play is completely dead when the ball touches the ground? I’m drafting up a scenario for more exciting beach football, and that’s one of the things I’m going to put in there. However, I don’t know all the specifics. Yes, kickoffs would be somewhat odd, but again not that bad. The onside kick would be used but would be a high floating kick instead (rugby-esque, there). On a standard kickoff, players would be lining the red zone to protect the ball from dropping inside it. Fumbles would be less detrimental, but also less dangerous and lateralling would be more common due to the protection against the live fumble.
One thing in rugby that’s also a key element is the knock-on, dropping the ball forward. Basically a forward fumble. This is illegal and has the same penalty as a forward pass. I caught myself watching live fumbles in the NFL and thinking that the other side would get the ball because I had played enough rugby to know. I don’t think that a turnover should occur in the case of a knock-on in gridiron, but a yardage penalty would be good.