The mod that got me into modding still has a special place in my heart. It was the Shockwave mod for C&C Generals. Shockwave absolutely could have been vanilla if EA deigned to finish the Zero Hour expansion. It was just pure polished perfection for that game.
It actually works pretty well by chance more than anything else. 1000 ft is a really good altitude separation between aircraft. 500 is a good offset for irregular traffic too. Multiples of 150/300 m are more annoying to work with and 500/1000 m would waste an excessively large amount of airspace.