The siege mechanic seems to me to be underutilized, essentially just being a way to allow multiple fleets to converge in one attack, but it could be interesting to make it possible to take over a city by laying siege to it for long enough for the population to run out of supplies and surrender before they starve.
It would potentially be useful as a way to take over a heavily defended city with fewer ships than would otherwise be necessary, with the downside that it takes a while -you can't use your fleet while it's laying siege and perhaps more importantly it gives your enemy time to react and the chance to potentially break the siege by attacking with their own fleet -or by laying siege to one of your own cities.
It could also be handy to make the 'steal supplies' spy action more useful by reducing the amount of time a settlement can hold out for.
