Right now, my concerns are more for the computer's sake than for players. The AI is not yet adept at building large fleets or performing adequate defense.

Guass cannon weapons platforms are too overpowered in the early game and too underpowered in the late game. They need to scale better. They need to start with a smaller amount of damage in the early game. Then, through research, they should be able to gain more damage and a faster rate of fire. They should also gain the ability to use shields and improved armor.

The hangers are too underpowered. For 4 tactical slots, I would expect at least 4 squadrons. I would also expect that a planet could host a lot more squadrons than 4 or 8. I would like to see a research option to research a huge hanger deck that could host 20 squadrons. Since this would be for late-game use to help defend against large fleets, I think it should take only 10 tactical slots.

I would also like to see mine fields in the game. I am thinking that in mid-to-late-game research, an option should open up to research a mine field generator. Once built, the mine field generator would slowly build a minefield surrounding the entire gravity well. The idea would be that the generator builds a spherical field of mines around the gravity well. The player should be able to choose where it starts building mines, and it should build in a clockwise direction. Over maybe 10-20 minutes, it would build an entire minefield. The point of the minefield would be to help defend against large numbers of ships. The mines should be strong enough to at least take out the shields of any ships passing through the mine field. As mines are used up, the mine field generator would slowly replace them. Friendly ships would not trigger mines, but would fly more slowly through the minefield than through normal space. This would also cost 10 tactical slots.

Finally, I would like to see some long-range ballastic missile batteries. These would be high-damage, long-range weapons platforms. They would carry maybe 20 missles and fire at a rate of 5 missles every 5 seconds. It would recharge 1 missle every 30 seconds. So, it would take about 10 minutes to fully recharge the missile platforms. The idea behind these would be to make them much more powerful than the gauss cannon platforms, but have limited ammunition so that people don't rely exclusively on these. They would be used to take out capital ships or other large ships that attack a system. It would help equalize the fight against a large fleet. In 20 seconds, they would deplete their weapons and then be basicly useless for the rest of the battle. But, they would deal enough of an initial blow to hopefully give the system a fighting chance against a large fleet. My only concerns with this type of platform are how to program when to fire and who to fire at. Ideally, they would wait for any in-bound capital ships to arrive and then target those. Their missiles should be able to aquire a new target if their original target is destroyed. That way they are not wasted. I think maybe 3 slots for each platform would be ideal.

So, a fully developed system with 35 tactical slots might look like this:

1 huge hanger deck with 20 squadrons (10 slots)
1 minefield generator (10 slots)
2 long range missile platforms ( 6 slots)
1 planetary shield ( 2 slots)
1 repair platform ( 2 slots)
5 gauss cannons ( 5 slots)

That would use all 35 slots and would offer some pretty beefy defenses against a large fleet.

A fleet attacking this planet would go something like this:

Initial ships (frigates) arrive, capital ships still in phase jump.
Fighters and bombers aquire targets and attack.
Initial ships start passing through minefield getting their shields knocked out and possibly one or two being completely destroyed.
Gauss cannons acquire targets on smaller ships and open fire.
Capital ships arrive.
Long range missle platforms target a single capital ship and open fire. Over the course of 20 seconds, they destroy one capital ship and critically wound another.
If the critically wounded capital ship passes through the minefield, it might die depending on the route it takes. If it goes through where other ships already passed, it might be fine because most of the mines have been used in that area. However, since it is larger, it may still take enough hits to be destroyed.
Gauss cannons, fighters, and bombers continue to pluck away at the frigates.
Finally, backup support arrives and ships engage the enemy. Ships have to get to the enemy, so they have to pass through the other side of the minefield at a slower rate of speed.