Wow, jorb. Those sprites look awesome.
Why not make the monsters NPCs who hang out way deep in Mordor (maybe in a cave somewhere) and go on "rampages" at semi random intervals. Each monster has an obsession, and seeks it out (they can see the whole map). The Dragon homes in on whoever has the most coins (hey, it's a dragon, he can
smell money from miles away 
), and kills the character or smashes the chest and carries away the cash to his lair. The huge ass ent finds the largest concentration of coal/blocks/boards, smashes the chest and a small area around it, carries off the wood to its "graveyard". The hill giant finds a pile of meat/sausages and eats it all, taking off some to his cave for later.
The first lair is randomly spawned, and some time after a monster is found and killed, a new lair is placed some way out into Mordor with a new monster. It's not too far out or too close, but not a specific distance away, so you have a sort of "ring" of possible cave locations. The old lair remains until the new monster is spawned.
As a result of this, you have to organize raiding parties (because the monsters are way too tough for even a very strong single character) and find (and you have to search a lot of Mordor for it) kill them, otherwise it might just decide to mess up your chest when you're offline. You can't just wait in town for it to come to you, either, since you never know what the new monster likes and where it will decide to raid, besides the lairs have pretty good loot which you'll have to look for (whereas you can follow the monster back, from a safe distance, as it zig zags back), and find before everyone else.
The loot would be too small to justify professional monster slaying just for the loot, but big enough to make you consider killing it at its lair vs. just defending your town.
We could have our very own randomly-generated player-driven Beowulf epic every couple of days!