New Vehicles are the easiest. The issue with Buildings is that the command panel menu bit can't be modded. The buttons are all hard coded. Vehicles however are just from a dropdown listbox.

Ah right... vehicles it is then....i never knew about the structure panel thats news to me

With the WIP builds (2.0.4.0 and higher) its possible to do more with vehicles.

You can make for example a crane that has a cargo hold. Previously new parts were not possible as the game only treats specific part ID's as being that weapon/etc. So if you tried to make a crane for the arabs for example it wouldn't work as the game wouldn't put the buttons on the screen as it wouldn't think that vehicle was a crane. With the latest build you can base a weapon on another weapon so the game treats your new weapon as if it was like one of the original ones allowing you to give other nations new weapons and make your own weapons with different stats (You could already make new weapons, but they didn't inherit technology bonuses).

So in the future there should be more posiblities.

Would it be possible to make vehicles that could travel on terrain that was previously untraversable? Climb/hover over steep slopes and mountains? Or even a subterranean unit like the Nod had in Tiberian sun?

You can only do whats currently possible like with any game. You can give Americans and Russians a chassis which is like the arabian hover one, but you can't make up some new form of transport that doesn't exist in the game.

Tiberian Sun was designed to have subterranean units, it wasn't added via a mod.

Ah right thanks for clearing that up House, was just curious.

Always looking for *tactical* ways to get round the Rows of Turrets-on-Hills malarky. Would be nice to have some vehicles that could.

I got idea of simple mod that gammers would like ( i think). Game clock that displays time that passed from the start, form of timmer. It would help allot during multi and single player. CHeers!