Alright here are my top 5 GI Joe HQ play features.
5) Big missile firing cannon on top. Of all the guns that need to be spring loaded in GI Joe, this is
the one.
4) In the cartoon, right under the cannon, was a door where the Joe vehciels came out of. I think that is straight out of Castles 101.
3) I loved the 83 Comm center with pictures of Scarlett and Cobra Commander on it. Also the cartoon had a large video screen so a back lit large screen and some smaller ones around it would be cool.
2) Jail cell. These can be either barred cells or my futuristic force field cells. Cause like others have mentioned, Cobra escapes would be a huge play feature.
1) Probably one of the coolest features of the Terrordrome, the 83 HQ, the later HQ, and the MCC is that they have places for the vehicles with fuel hoses and repair bays. If the Joes are at home, they'll probably be fixing something or tweaking something.