I’ve only thought about the craters  but haven’t actually done it nor do I know how and as for the aura, I haven’t tried it with anything yet. Craters, I mean actual holes, like those created by the terrain tools in Atlas.

For this the projectile can probably spawn an entity (the crater) with an aura (slow units in the crater), have you tried that? What would cause performance issue?

I used to think about the craters left by explosions since, to me, they serve as a measure of firepower, but it seems they would cause issues for performance and gameplay.

In the code there is a TODO comment that hints that terrain bonuses already existed in 0AD, but got removed .

Thank you, I'll keep that in mind. I have a type of rocket launcher that can fire anywhere within range, you can simply click on any point, and it will fire regardless of whether there is an enemy there or not.   I really like the small details, like applied forces or weather conditions, that you and others are experimenting with. I hope someone does something with water and terrain as well.