I thought the arty system in Dawn of Aces was pretty cool. Only certain planes (two seaters in DoA) could call in artillery from fixed bases. First you'ld call for a spotting round, then you'd call in range and aszmuth corrections. Once you got it spotted in, you'd call for barrage fire, again calling for corrections occationally to walk the barrages around the target. In addition to fixed batteries (which can be destroyed, and are captured when the base they're tied to is captured), allow a player to spawn a six-gun battery of towed arty. It would be a perked option, of course.