Since most ATVs handle like a wheelbarrow full of bowling balls going down a flight of stairs, I think the engine size should be limited to 49cc's or less.
Seatbelts, roll cages and ejection seats should all be mandatory items. Riders (drivers?) of these ill-handling pigs should be forced to wear bright orange t-shirts that say "I'm with stupid".
Barring that ATVs should all be melted down. Have you ever got stuck behind one, as it tank-slaps down a narrow dusty trail?
For the good of mankind BAN THE ATV!