Amos did some really nice drawings of Trebuchet - and I agree, Ulm should be able to recruit those. They have a giant siege bonus and a large area of effect attack that fires every third turn and causes fear in an even larger area (can you do that or does the area of #secondaryeffectalways have to match?).
The flaming pig ammo is what you get when you cast flaming arrows
I would be averse to tossing in some cannon and arquebus, but they should show up in the late era, if at all - and they should be seriously limited, as in WFB. The only nation that might conceivably get them in the middle era would be T'ien Ch'i.