An AI hook is actually my top priority for projectiles, since it defines most of their behavior and many vanilla aiStyles rely on the projectile's type. Hooks for hitting an NPC is also planned (and also required for one of my goals for the NPC hook update).I'm not sure what you are planning for projectiles, but the number one thing I'd want is the ability to have them run their own custom AI loop instead of being forced to use one that already exists in Terraria. I've been able to "fake it" pretty well so far though by reassigning the aiStyle of another projectile. Hooks for hitting an NPC and hitting a tile would also be nice if that's workable.
There's already a hook for hitting a tile. The KillTile hook is actually for whenever anything attacks a tile (I find that name a bit misleading but it's what's in the source code). If the fail parameter is true then the tile was just hit and not mined.