if you want to do away with kickoffs just give the teams the ball at the 30 and be done with it.
thousands of youth leagues do this and it works just fine.
the 4th and 15 nonsense is just that. you don't make up downs.
if you remove kickoffs you remove onsides kick options. that's just part of the deal.
if they want to extend games and make late game play more interesting stop the clock after all first downs in the finals 5 minutes of a game.