Rather than adding new stuff, my guide focuses on making what is already in the game look better. So there are plenty of mods that improve the look of the landscape and dungeons, but they don't do anything to make them more unique or interesting. You may want to check out bevilex's guide instead, which focuses on just trying to make the game look good as possible without worrying about how much it differs from vanilla Oblivion- https://www.nexusmods.com/oblivion/mods/47591/? For leveling, Oblivion XP Update and Realistic Leveling seem like the two best options. I use OBXP myself, as I mention in the short gameplay modlist linked in the main guide. For landscapes, you've got Unique Landscapes and the HESU Mod Collection to choose from. Then for dungeons, you can use Better Dungeons and/or Snus Dungeons.