Joining battles / Coordinated assault and nobles giving quests
In my experience when attacking a site, there is a good chance my military gets wiped out because in some random goblin pits with 20 population, just happens to be a goblin present with slightly higher military tactics skill than my commander. (Also i want to mention it would be nice to have the ability to scout for more information about a site like number of other creatures, and maybe even the military skill of defendants.) So instead of having to fight every battle yourself, have the ability to join ongoing/future attacks. In the "c" screen you can already see rumors of moving armies so i thought, for your own civ, there would also be a list with ongoing / future attacks which you could join.
If more armies could join an attack, then what if your king could order you, and other fortresses, to join in coordinated assault? Like a messenger would come and told you to send a number of troops by a certain date to a location. This location would be a military camp with armies from other fortresses as well and after all armies are present they go on to attack a site.
And so i thought if the king can give you orders what if he (and nobles) could also give you quests like in adventure mode. These could be same as in am or more suited for fm - like reclaiming conquered site - or perhaps both.
Strategical importance of sites
Last suggestion, this one is more suited for the economy update. Not all sites are equal and some have better acces to iron and flux stones, more valuable metals, better conditions for producing food etc. So these sites would provide a lot of arms/wealth/food for their civ which would make them a prime candidate for conquering/razing. Conquering/razing them would have a serious negative impact on their parent civ but of course these sites would be very well defended.