Good job
I'd like it if you removed the bomber and helicopter, and instead put one more biplane due to the actual timeline of technological advancement. AA turret needs a bit more work, as in platform stuff.
Print arrows should be removed, as they make a negative impact on the display, and if I am correct, doesn't need to be there since the island should be close by.
Carriers require boats. to travel to island
Sandbags need changing. Just using round corner bricks don't make it look like them. If you want to keep it like that, you should probably change to grey to represent concrete, or use stacked 2x2 round bricks.
Classes are okay, but for the Grenadier, you should put the TF2 Pipe launcher instead of three grenade launchers, and put two stick grenades.
Main capture point should LOOK like a "capture" point, and should look somewhat like a strategic point for some type of war, instead of having a "Here!" sign.
Island is very good, although, what's the brickcount of that itself?
OVERALL: 7/10 on general design/concept
9/10 on building. I don't build very well, and this looks great.
For now, you are in [SG].