The problem with credits, is that you can get an obscene amount of them, even if 1 credit = 1 point of damage. I've kind of abandoned this suggestion, but coming back to it, I've realized I managed to over-complicate it. It'd be much better if it was just a matter of the homebase being vulnerable whenever 25% of a faction is online. There would be more details, but that's the general jiste of that idea.Here's my simple solution: maintenance.
Basically homebases should have native credit storage. Homebases are invincible if they have enough credits to pay for each damage unit inflicted.
Basically the system philosophy goes like that:
1. There must be at least one faction member who logged in half an hour ago to make their homebase consume credits. It will prevent players to destroy homebases while faction members are offline yet also preventing losing faction members to log off and wait until the attack is gone.
2. The amount of credits consumed each time depends on combined mass of a homebase (including docked entities, cargo and stuff) and damage inflicted. Amount should grow exponentially with mass and linearly with damage. That will allow bigger factions to not care about little fly attacking their base as they have vastly more resources. On the other hand it will prevent developing factions from being utterly destroyed by random people. We all know that we shouldn't fly something that we can't afford to replace.
3. All faction members must be notified of their homebase being under attack. They will get who's attacking and amount of credits they lost during first attack and amount of credits left in storage. If attacker used even bigger weapon, faction members must be notified again and state a bigger sum of credits. Only evergrowing credits lost due to certain players should be reported. Everything will be reset after an hour since attack has been stopped.
By doing that relatively complex operation, we can notify faction members about enemy attacking and evaluate whether they should worry or not (amount of credits lost / amount of credits left)
I am done here. Hope I covered everything.