Maybe the real thing lacking from “endgame” is any obvious incentive to break a contract? If you already have a full company, all on (or close to) level 11, and enough crowns to buy whatever you want (but only if you make room for it by selling off some of your precious loot), you only take contracts on to start with because they save you the time and boredom of having to find some stronghold to attack. But if things happened to make you want to break a contract …
I don’t think there should be “impossible” enemies, since that would make your efforts futile. And the mere promise of great riches hidden within a package is not a good incentive once you’re already over-laden with loot and riches.
But what if you became caught up in an internal struggle between the townsfolk and the nobles of an allied town? What if the peasants asked you to protect them against other mercenaries who had taken pay from a noble house to act as bandits? What if some “stronghold” a noble house had paid you to raze turned out to be occupied by some semi-starved dissenting faction? What if you were paid to rescue some noble’s daughter from bandits but she then revealed to you the horrors of her home life and begged you to protect her from it?