Well, lets go by the existing sites and buildings to see what might be needed as well, for the current behaviors at least.
Hovels and apartments - Residence types, with apartments allowing multiple families, and having multiple stories. We might need to add room information to keep families separate (which might be as easy as in Dwarf Mode, define a room from the bed and the table). The food barrel might need to be defined.
Shops - A shop type, obviously. There's multiple variants, so there needs to be a way to create variant buildings easily. The shop tables need to be defined, and the money chest and cashier table as well. There needs to be some definition what the shop sells, which I'd rather not have hardcoded (Imagine a large unroofed marketplace with multiple marketeers with three shop tables each who sell diverse goods). A tag to tell the game that shops and other buildings are important for legends and need to be named.
Meadhalls - A site government type. Might need a definition for where the position holder prefers to stay.
Keeps - A civ goverment type. In addition to the above, might need to attract soldier types.
Dark Towers - Residence and site/civ government type, I believe, so it needs to be able to put multiple "type" tags onto a structure. A way to randomly create rooms, corridors, multiple stories, etc.
Temples - A religion (and perhaps religion government?) type. Needs a way to specify engravings, and liquids. Also needs the variants, and sphere alignments for the variants. Some way to say that temple variants are mutually exclusive, while shops aren't.
Cave entrances - need a similar random algorithm as dark towers, and need to go as deep as needed to reach the first caverns.
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Towns - Changing tiles by population. Replace buildings (hovels) with larger buildings (apartments). Number tags for buildings! That'll actually solve the temple and shop issue above. Hovels and apartments are as needed, others get 1.
Mountain halls - they're weird right now, but definitely need a tag to be built in deeper, stone layers. Ideally without breaching into the caverns. Also, a tag tag to prevent embarking on them.
Forest retreats - nothing special to see here.
Dark fortress - As mentioned above, a number tag for buildings. There's three dark towers, and potentially a temple.
Ruins - Ruins should be defined in the other site definition, come to think of it. That is, a tile and color for 0 population, and perhaps a reclaimable if ruined tag.
Cave - need a tag to be placable in worldgen rather than being created by settlement, if it goes that far. Could be cool for other adventure sites. Also would need a way to populate them with creatures (custom biomes? Creature class lists? Undead only?)