I was thinking that there could be special planetary modules that could be installed on a planet and if you guard that planet it gives you a boost to you stats.

Here's an idea:
Communications Tower
Planetary Structure Size: 5 Research: 2 Guard Boost: Scan +5 Planet Limit: 1
This guard boost effect would only be effective if you're gaurding that planet( which holds the module)
If you have any of other Communications Towers installed on other planets, then your guarding boost is is amplified by the number of Communications Towers (Formula - N X 5)

What do you think?

Decent, although Scan +5 is laughable.

Something (perhaps dealing with the moons and rings and shields coming up) more generally useful wouldn't be bad. With the shields coming up, one that could increase your shields and add some sort of shield to the planet (such that attackers must get through the shield as well as the population).

I see a lot of possible pitfalls and holes with this and also some useless thoughts, but not a bad thought that could form a generally nice idea.