i read this in an other mod (i think it was res gestea or EB), but i thought it would be a nice feature to add to SPQR.
each faction gets a 'treasury' in its capital (like some sort of tempel) where, let's say, half of its total amount of denarii is kept. so when you conquer an enemy's capital, you gain half of its treasury and thus giving the faction a financial blow.
the idea is that every income you receive each turn is split in half, one half goes in your treasury and the other half goes to your amount of denarii which you can use directly. that means the half that goes in your treasury, isn't available unless you destroy your treasury which, off course, will have some negative effect on your economy (i haven't quit figured out yet how to do so). if you destroy your treasury, it's pretty much the same as an enemy sacks your capital. that should only be the case in emergencies.
i don't know if this is practicable, cause it would seem a bit unfair if you conquer a capital which is only guarded by 1 unit or so. so every capital should be filled with units to defend it properly. it has to be a challange.
i don't know how the guys from RG or EB made it functional, but i liked the idea: if you just started playing a campaign and you could sack macedon's or the greek capital, think of the loot that will enrich you!