Only problem I can see is they are persistent... This can only be done on a server side basis. They would have to loop and follow along then stay there for 5, 10, 15 minutes. That's a long time, and then if you get a lot of cons crossing previous contrails it gets really complicated.
All this the server has to track. All this has to be transmitted to you. Lag would introduce other issues, as well. Position reports would have the contrails 500 yards off from the plane originating them, or possibly in front of the plane...
It's nice for limited scope games, or offline games... But for AH that's a tall request. One I would love to see, but to do it right would require a LOT.
To offset the in-game/server limitations, how about adding the option to the film-viewer
instead of the game? This would allow contrails to be seen in films, perfect for
taking screenshots and creating movies, the like, without the server needing to do the
work.
This also helps players with low end PCs, such as myself, to view the eye-candy later
without having to deal with a low frame-rate. Currently I run the game with anti-aliasing
set [off], but turn it [on] for when I want to take screenies from a film. I'm trying to point
out that it
may be possible to do something similar,
and worth the time.