the mechanic of the map could work.
the idea of this would be a shield around the hoth base would make sure that the at-at's and the at-st's cant be dropped right infornt of the base entrance till the shield is down.
they would have to be dropped on the outer beyond the shield. now as we all know in starwars you are able to walk through the shield so the walkers will be able to fight inside the shield. how ever aircraft would not be able to fly through it. (you might be able to use the you are leaving the battlefield damage mechanic with this for fighters)
then once the shield is down maybe you could stop the at-at's and at-st's from spawning in the ISD

just my 2 cents

Compulsive doodler

I came up with a similar map plan for a Mon Calamari map a while ago.

Very rough initial setup

perials objectives would be first off.

1. Grab a Tie figher and head out of the SD

2. Destroy Shield Gen 1 and 2 to disable the shield which is defensive barrier between you and Power Generator red.

3. Now , this allows the imps to dock their transport ships onto the docking bays of the Power Generator RED and enter the domed building to destroy the generator. You dont HAVE to dock ..the Power Generator RED would have pylons/aerials that stick up thro the dome ..so this means you CAN disable the RED generator in your TIE fighter...however, it takes longer to destroy ,since you arnt getting at the base of the generator.

4. The Imperials have prevailed in destroying the Red Generator and now the RED HANGAR  shields protecting the main Mon Calamari City Power Plant are disabled and the Imperials can now begin to dock into these hangars and try and make their way to the Reactor .

5. To get a much easier time of battling thro the power plant corridors, the imps could take care of the GREEN Power Generator and disable the GREEN Hangar shields ... thus getting 4 entrances to the Reactor room.

6. Once the Reactor is destroyed then the Mon Calamari Cities are Powerless. Imps win

On the flip side... If the Rebel/Calamaris and Quarrens destroy the Star Destroyer ... they win  . Either this , or a time limit could be introduced so the Rebs,Cals and Quarrens just have to hold out against the onslaught for a certain amount of time.

EDIT:

After some discussion we thought it may be better to have it an Attack(imps) v Defence (cals/rebs/que) so that the aim for the defenders wasnt to blow up the SD or board it. They could disable parts of it by destroying parts of the hull and damage the imps that way.

The main aim of the map was to have everyone dogfighting for say 10-15 mins , then an all interior corridor fight (much bigger than the bf2142 titans) for 10-15 mins.

Tortel, a gifted map designer, made such a map for GC back in the day. It featured a static VSD hovering (50 metres or so) low over a rather flat and desolate battlefield. On the planet were several Rebel outposts as well as a city, and the Imperials were to land and capture it all. It was rather splendid, but released too late to ever be played online (GC's glory days were long since past).

I had a similar idea where you had the surface of a planet at night, then an opaque 2D surface that looked like a sky from beneath and the planet's surface from above. The sky-box would be stars so that it'd look right both from the surface and from space. You could fly through the opaque surface, of course, and even fire the ISD's weapons through it. Only problem is that the ISD's heavy turbo laser battery is dorsal, so the ship would have to be at a 45-90 degrees horizontal angle so that its starboard or port side was pointing planet-side.

     [Perhaps a bit too frank and picky, sorry]
With all due respect, I don't know about this "destroy this and that generator and then this and that blast door and reactor and cooling system and cafeteria and zoo and that-utterly-unimportant-location-that's-included-just-because-it-was-in-that-one-movie-for-two-minutes and whatever the Heck else to make everything go BOOM!" type of map that everyone seem to love these days, though. Fun enough on Hoth to destroy the generator and then take the base, but it gets unnecessarily cumbersome and frankly a bit cliché-ish in the long run. You want something destroyed, you destroy it. You want something captured, you board it and capture it. Plain and simple. Not to mention that if you want something captured, you don't destroy its reactor. Not only would it leave your prize utterly useless, it'd also make it untouchable for centuries or millenniums due to the catastrophic Chernobyl-style radiation leak that'd follow the destruction.
    [/Perhaps a bit too frank and picky, sorry]

As for Hoth, just make it a regular ground map. The only interaction with space should be the ion cannon randomly firing up in the sky and perhaps some scripted transport/X-Wing departures, just for eye-candy's sake. Nothing more. Imperial reinforcements should come from the Imperial Drop Zone, not from space. Seeing that there was no fight at the Drop Zone or on the way down, this would make far more sense.

Imperator class Star Destroyers can't go into a planet's atmosphere without crashing. You'd have to use Victory Class ships or something even smaller. Heck, the Victories can even land on planets' surfaces!