1. Orc Warlord (size of giant, with plated orc armor) Loot: Heavy armours, 2-handed weapons
2. A Daedric Lord ( Maybe like Sauron from lotr AKA: Battle-mage) Loot: Enchanted Light/Heavy armour's/weapons [for warriors/archers who dont train wizardy]
3. Demon ( im sure they'd come up with a look) Loot: "maybe some new type of Gear"
This is all i can come up with rite now, but even this would give us way more things to do, and i dont want these to be your avreage monster fight, more like a challenge, where you need to get prepared for it, maybe gear up in certian armor, certain weapons.
feel free to post your boss type monster you would like to see