I think one possible take at this would be to tie in quests with events and mega-events.
Let's say there is an event: "brigands appeared in your kingdom and prey on caravans" - than the sovereign can either live with the losses, train troops and guard caravans, train big army and look for/eliminate the threat or pursue a quest path 2 or 3 quests long to find and deal with the threat adventurer style.
example: 1 quest defend 1 caravan with your heroes
2 quest find the lair of brigands with information from quest 1
3 destroy/recruit/mind control/... the brigands in the lair.
more elaborate and longer quest paths would be possible for other events:
appearance of a volcano,
army of darkness is attacking,
wood spirits are seen in the woods...
imo it would make the game more flexible giving the sovereign more possibilities to deal with different situations and make sure the adventuring tech will stay useful throughout the game.