System blockade / bombardment

Don't the Cravers currently use a similar function with stealing population as their unique faction trait?  I wonder how this harassment style tactic would play in with other factions.  You obviously seem to be talking about this from the cold war status of the game, but I wonder how this would be manageable when even in a cold war state (when you're not allowed to overtly attack an empire, which is exactly what this kind of tactic would be), how this kind of tactic could not be considered as an act of war.  But I do like the idea of having planetary bombardments (while in a state of war) which could be used to weaken an enemy stronghold prior to initiating the actual ground invasion or to completely destroy a population, completely removing the colony in those times when you don't have the manpower needed to conduct a successful invasion.  But such a gameplay tactic should probably require special equipment on your ships for obvious balance reasons.

I haven´t played with the cravers yet, ill give them a go soon. The way I imagine the bombardment idea is not as a cold war mechanic but as an open war one. What I would like to do is to be able to hindered my opponent production, science and population without having to invade their system since it comes a point in which managing to many systems becomes a bit annoying, also, since conquering a planet alters the composition of your population, you may not want to do that.

Regarding balance, perhaps it should be a special ship, one that you cant use for space battles, that way you would really have to weight in the pros and cons of building a bombardment fleet and having to protect it.  Also, planets could have a way of defending against this attack, maybe bunkers or AA cannons, each planet could have an orbital defense coefficient based on buildings and tecs, if you bombardment fleet doesn't have a higher one, based on the amount of bombers present in such fleet, then you cant damage that specific planet / system.

I imagine being able to have a main fleet going for the yugular of my opponent while at the same time using smaller, more fragile fleets to harass their weaker systems. This could come in handy when fighting against multiple enemies or assisting allies in a war.

Definitely needs to be a specific ship designed for bombardments and particularly weak against attack.

Means you would have to have a big enough fleet to carryout the strike and prevent early harassment because that's not what its meant for and easy to counter unless you have a large fleet to protect.