Going for 4th and 15th from your own 25 seems extremely punishing if you don’t convert. If you don’t recover an onside kick the other team would have the ball probably around the 50. Id like to see this play from your own 35 instead.
Teams are most likely only going to attempt this, when they are far behind and would lose anyway so they won't care about the outcome if they fail.
This was the rule the AAF had. You also could only do it within 5 minutes of the end of the game, or if you were trailing by 17 or more at any point. I liked it because it gave the trailing team a small but not impossible chance of overcoming a large deficit at the end of the game.
The only downside is no more surprise onsides. On the other hand, Ambush will now be a signature play that no one will ever be able to repeat :)