I think there's easier ways of accomplishing the same goal without overcomplicating things or resorting to gimmicks like 4th and 15, etc. Just give the kicking team the option of:
a) choosing to give the receiving team the ball on the 20, or
b) onside kicking from the 30 or whatever. If the ball travels more than 25 yards, then R gets it 25 yards past the kick (provided there was no R penalty) or the dead ball spot, their choice. This would just mirror what we already have for free kick out of bounds. Keeps the on-side kick in play, doesn't really change things when they're used, cuts out a dangerous, almost always anti-climactic play (not quite as much in NCAA with the 25 yd line touchback, which ironically IMO is encouraging short/pooch kicks but would like to see actual data), and lets everyone move on.
Frankly, for all the wailing and gnashing of teeth on this issue, I don't think we'd miss it all that much at all.