for example, we can breed our own monsters or construct our own golems, then we could bring them into battle, these pets should be customizable and requires some maintenance. monster should be ragularly feed to make them grow and stronger. while golem requires repair cost.
but since they are gold sinkers, they should also prove very useful in your quest, for example, golem should be able to carry extra loot for you, or you could ask your golem to pickup gold. and depending on your customization, they can either be a tank or a pack mule.
while breeded monster should affect or help players with different builds, for example, you are breeding an ice elemental type of monsters. these monsters should gives you extra buff on your cold skills or vice versa, gives you high resistance to cold damage. something like this would gives us plenty of room for different builds not just gears.
and lastly monsters can be traded or sell. monster should not stay forever. they can die and mature and so we can breed new ones. and upgrade our dungeons.
Stronghold should also gives us other perks. (I'm sure you guys have other ideas)