I would take some inspiration from the Bitterblack Isle from Dragon's Dogma - Dark Arisen. For instance while you are killing trash, there's a chance of a rather hungry monster joining the fun and charging all over the place. It would be untankable and require people to be quick on their toes to avoid being eaten. As for layout there would be multiple paths and approaches to each boss room. Some quick, some winding with extra monsters and loot. Of course if there's treasure chests a few of them should be mimics!

With regards to the mini boss encounters, depending on kill speed there may well be more than one to really mess with the party. The main boss rooms could have things like ballista's/cannons that can be used to temporarily stagger the boss. However not using these would lead to extra loot from the extra challenge.

If the boss is a tall monster then perhaps an arena with balconies around it that allow you to hit it in the face rather than stabbing it in the toes. This would do more damage than staying at ground level, at the expense of splitting the party and making healing an issue. The tank would need to stay at ground level in order to keep aggro, meaning that the healer would need to be with him rather than chasing the DPS around up top.