Just a rough suggestion, so feel free to shoot it down/discuss it civilly...

When you invade planets, two out of three of the possible invasion triggered events are negative (-50% def, or +100% build cost). So, I was thinking of a newer, positive outcome event.

Liberated Townships: The population of the planet are overjoyed to see you, and their happiness creates a boom in trade - the planet gets +X% on a random production trait for some period (where X is open to debate. I was thinking 5% for a week).

There could also be a followup to the uprising medal, which could be something like "Liberate 10 townships on planets during invasion. Reward: TITLE: the Liberator"

I was tempted to put something like liberating the towns gave you something similar to rescued prisoners instead of the production boost, but I feel that footballs would become even more well used in order to attempt to get RPs out of them. Still, you can comment on this idea too if you like! I'm kind of "meh" about that one. Maybe it could be an artifact that is the reward to the medal... Liberated townspeople - rewards you with 6 (maybe 10?) rank points.

Sorry, that suggestion kind of got in to a ramble at the end. But hopefully it's clear enough!

The event only have a chance of happening on planets with a high amount of defence and attack at least 10K-20K + ? in each, i.e your setting them free from enslavement, so the planet must have a high amount of attack and defence on it.

There already is one of those, it's called improved invasion.
Now I'm not entirely sure if it's a random event or related to something else, as it was gotten when someone else did a invade, however from what he said, he have no clue what caused it as well so I'm going to guess it's random proc.

I can't quite remember the effect exactly, but I think it's +10% or 20% all production for a week.

It's +50%, and is a passive ability unlocked via the Lone Conqueror Medal.

Yup, what Darth said. It's an unlocked ability that involves not being in a legion, so I know that there will be some out there that won't want to unlock this (at least immediately) as being in a legion is of more benefit to them. The random invade event gives a small chance to get the equivalent.

Once you've unlocked it, it'll work every time, regardless of whether or not you're in a legion.

Getting the medal only involves hoping out, stealing the planet, and back in fifteen times.

Yes, sorry Darth... my post wasn't clear. You've got to be out of a legion to get it, but you're right, once you get it, you can be in a legion. As I say, it's just a suggestion to have an event that gets something similar without hopping out of legion to be able to unlock the other one.