My thoughts on these unit suggestions:
I understand these are just food for thought, so don't think I'm trying to bash on anyone, just giving my thoughts:
Light tank:
I like this idea. I think that sounds like a balanced unit. My first thought was it was too long reaching, but the fact that it does very little damage makes it useful.
Assault tank:
As it is, this tank is not very usefull. Impressive, but not better to buy then either the Heavy Bot or Rocket Launcher. It would be too susceptible to artillery fire, and as you learn if you ever try to use your heavy bots on the front line, too weak to hold up long. I think it still may be balaced if you give it a 2 move instead. It would still have a reach less then the heavy bot, but would be an enticing purchace because of the damage it does. Of course, the fact that you could load those beasties on land transports would make them Wayyy powerful. You could load and unload them on the same turn with 2 move. With that in mind, maybe they shouldn't be aloud to enter a transport, or better yet, take up two slots. If they couldn't load on transports at all, that would severely cripple them on water maps.
Mines, obstacles:
Not too bad. I don't see why someone would buy one of those instead of two LAVs though. Any way to make them slightly more useful to make them more enticing? Maybe make them a mini-tower and give them 2 damage? Or would that make them too tough? I don't think so myself... They would have only one hitpoint more than a tank, but no move at all.
Maybe if there was a minelayer unit to go along with it, so they could be picked up and moved... (take away the damage in that case).
Fortresses:
Sounds descent to me. I'd hate to see one of those pop up as a guerrilla unit in a forest... At least with their price, you wouldn't be finding two of those in one place too often.
Cruiser:
Sounds good to me. Well balanced as well. I don't know if you'd be finding too many Destroyers being made any more. For 1$, you get +1 range, +1 damage, +1move. Maybe knock down one or two of those abilities.
Sub:
I don't think that they would be useful at all. With their move, how are they a threat to transports and amphibs? Now they could be cool if they could be invisible until someone gets within two or three range of it. They would have to be visible when they are built, but are hidden until they show up ready to torpedo a hapless transport. That might add some fun.
Heavy Bomber:
I'll have to think about that... The fact that you could put three of those on a city would make them Naaaassty. Of course, that would cost 21$.
Hehe, each of those would be as expensive as an aircraft carrier themselves.
Helicopter:
Way too powerful for their price. 6 moves unaffected by terrain? Ugh! Maybe make them 3 moves. They still would act like they had 6 moves over desert or forest. And another thing: An airplane bombing a helicopter would be a pretty funny animation

.
One more idea:
How about a helicopter transport as an addtional unit?
Move: 4 (as a helicopter, unaffected by terrain)
Fire: 0
Range : N/A
Hits: 4
Cost: 3$
Can load one unit inside.
That could make for some pretty fun sneak invasions over mountainous terrain. I'm not quite sure of the balance of this unit, maybe it should be more expensive