As well as the existing forts, ruined towns, etc. I feel I should point out that there are two other existing locations that could lead to possible homes -
Spoiler Depending on your actions, High Hrothgar or Sky Haven Temple
It would be great to be able to upgrade either of them with full facilities (including Smelter and merchants) after completion of the main quest.
I do like the idea of a castle/fort, though. So long as it is a "one-stop-shop" with
everything. It's annoying having to fast-travel to three, or more, locations just to do my enchanting, smithing, smelting and trading. Especially trading. It's unnecessary busy-work to have to travel to several cities just to sell three or four items because each is worth the max gold of the merchants (I shouldn't be forced to invest Perks in an entire constellation just so I can sell the loot from a single dragon in one place).
What would be even better than a "simple" housing DLC with a castle/fort/town would be a new faction for you to lead. The difference is that you build it from scratch in your new home. You hire NPCs (some of whom are Followers) who then give you quests to upgrade your home and then, once it's fully upgraded, give you Radiant Quests which could include defending your home as well as acquiring new decorations (like the loot on display in the Flagon).