One of the Oblivion's greatest flaws, and something this mod helps to alleviate, is because the simplistic and predictable nature of magic and enchantment, there was little to surprise you. From the beginning, you could see that most spells would just scale up the effect and mana cost, and most equipment would have the same new modifiers. This mod helps fix this to a degree with the secondary spell effects and set items, but I think it could do even more:
1. More items: Let's face it, one of the largest factors in RPGs is collecting all that epic loot. To increase replay value, there should be even more set and unique items, and if they aren't already, randomize their locations to a degree. I just love being able to go into a dungeon knowing that I could find some powerful artifact I've never seen before.
2. More hidden loot: I like how there are occasionally items hidden out of ordinary reach, usually with an orange light around them. Unfortunately, they're rarely anything more than some potions and a perhaps a soulgem, so it doesn't really reward your sharp eye. I recall one of the Oblivion overhauls had a mod that hid powerful weapons in completely out of the way places in random Ayleid ruins, and it would be nice to see something similar.
3. More enchantment effects: Not sure if this one is possible or not, but I'd love to see some non-standard enchantments on items, especially as set bonuses. Things such as bonus spells you get when you have the item equipped, increased spell effectiveness(which can already be done through endurance), damaging auras ala Holy Fire from D2, and whatever else you can jury-rig into the game.
4. More interesting spells: Things like daze and freezing were a good addition, but I'd like to see even more. Perhaps, things such as self-polymorphing or corpse explosion spells. Also, more things like Frostfinger's tomb- rare spells that you can only find in dungeons.
5. Unique monsters: don't know if this is possible, either, but try giving enemies a small chance to be more power versions than their standard type, like in Diablo 2. They'd have more health and stronger attacks, but much better loot.