Well, Harold did come out in a video interview and say that he realizes the fans would hunt and kill everyone involved if they didn't keep the original car. I could see Ray and Winston being uncomfortable with the newbies taking their baby out for joyrides though.
If I recall correctly, Aykroyd had all manner of vehicles converted into Ectos in his "Hellbent" draft. The one I always wished we could have seen was his "Ecto-50", which was apparently a truck based mobile containment unit.
I wonder if we'll get a nod to Ecto-8 from the game.
Just for giggles, I'd have to think about it though. I've heard the HHR suggested before and for some reason it never sat well with me as a good replacement. It doesn't seem big enough. In contrast, a more modern large ambulance lacks the sleekness of the classic Ecto. I saw a few suggestions for a new Ecto on another site, I don't remember the car types though. Let me go double check. In the mean time, for anyone just wants to know what an HHR may look like as an Ecto...
Here's a link with a bunch of people's different car conversions into Ectos.:
http://www.gbfans.com/community/viewtopic.php?f=13&t=10949&start=0