I like the map idea, though I think it'd be better to have it take place on Mon Calamari and have the island be occupied by the Mon Calamarians.
I really like the impending storm idea. It'll do a lot for the atmosphere.
What the problem will be is that on island maps, players are free to land where-ever they wish. It's not easy to force players to land at specific locations without making the restrictions too obvious (for example by barring all but the landing zones with cliffs). It's hard to defend an island when the attackers can land anyplace.
Perhaps if there was a radar network that saw where ships landed, and the Rebels had minor fortresses scattered about and fast vehicles to move infantry to them with. Or if the Rebels had swoop bikes so they could patrol the shores and spot landings.
I think instead of turrets, as said, there should be fortresses along the shoreline, about 50-100 metres in-land. These fortresses would be little more than a few low walls, a bunker or two, and some E-Web emplacements and turrets. When the Imperials land, the Alliance can rally the necessary number of squads to the nearest fortress and attack the island from there.
I'm not sure about these fortresses being control points, but I think their firing ranges should almost reach each other so that you can't simply run past them if they're fully manned. Perhaps the Empire could have an artillery ship or two of some kind (maybe a couple of Firehawkes or MIAU hover artillery platforms? For details on them, download the manual for "that other mod").
Also, I think it'd be an idea to give the Mon Calamari underground tunnels with which to get easily from fortress to fortress. Maybe there could be tram on rails, too, to get really quickly to where you are needed.
And yes, lots of foliage and trees is a must, then open ground in and around control points.
Sorry if I'm being annoying tearing your map apart like this. I'm only doing it because I like the idea.
"One of the bitches actually gave birth while she was attacking, and her puppies joined in on the carnage."
--the awesomeness that is Boatmurdered.