idea for a campain. please add on.

[Rukelnikov]

The times we did this, soon rather than later will the party try to get an army of bandits, thugs, monsters, or what have you. Usually go to a bandit/goblin/orc encampment and convince them to join/follow them either thru negotiation or strength (once they staged a high security prison break which was very fun). And from there its taking/razing hamlets, towns, and higher and higher, since its unlikely they'll go from having conquered nothing to having conquered/razed the entire world (if it so happens they manage to, in my group it never happened but some of the parties did conquer/raze many kingdoms).

You should think about how you are gonna handle things once the party already rules over sizable lands, if they take over a kingdom with gold mines, how are you gonna handle that? We devolved to something akin to a 4x style for the most part, the lands they had determined how many troops they could feed, and arm, and this was in turn part of what kept the story going, they had more troops than they could feed so they needed to take over food producing lands, and while they had money to "buy" arms and armor for a lot of troops, there just wasn't enough iron in their lands, so they needed to find a source to arm their new troops and stuff.

The other thing you need to keep in mind is how you'll handle armies combat, the time we did this in 5e we tried using the UA rules for mass combat, but ended up modding them to the point they had little to do with the original rules, still they served as a decent starting point, and I'd recommend them if you haven't done this sort of things yet.