I recommend Cynn's Honeyside for fans of Riften: https://www.nexusmods.com/skyrim/mods/85697 Colossal downstairs trophy room and armoury, with room for every single unique item in vanilla, Dragonborn and Dawnguard. Racks and mannequins for each and every guild reward (ie. The Staff of Magnus, Guild Masters, etc.). All Thieves Guild items, Daedric artifacts, and an optional downstairs armoury filled with weapon racks and mannequins. Upstairs has lots of room for unique items too - lots of bookshelves to hold your book collection, dragon priest masks, dragon claws, and other unique tidbits like Ysgramor's Soup Spoon and Exquisite Sapphire. Could not recommend this house enough if you're a die-hard treasure hunter and item collector. It has everything you want and more. Not only that, the developer actively listens to feedback and acts upon it quickly if a bug is found or something is requested often enough. It isn't a private castle by any means, but I think you might be able to overlook that if you're only interested in collecting unique items.
