Of all things to add, buildings comes at the very bottom of my list personally. New civs? Wonderful! New units? If we make new civs, this is a prerequisite, so sure thing! New Techs? Goes without saying as well for UTs! New buildings though? Erm, to be honest, Im so stumped, you can cut down a whole forest and you wouldnt be as stumped as I am.
The only building I can think of is the Mercernary Camp. Here, you can train your main UU without the benefits of conscription. You can also train 2 other UUs from another civ similar in culture to your own. Of course, these units will likely be less usable in your hands than the civ that gets them. For instance, the Turks could get Mameluks and War Elephants. However, No mahouts or Zealots is very staggering. The Huns for instance could get Mangudai and Huskarls. Yes, Mangudai benefit from a cav archer discount (interesting and true. Check it in the tech tree during game as hun in castle/imp age. It works on War Wagons too!), but they fire slower than the Mongol ones without speedy fire. As for Huskarls, they just cant compare to the original of the Gothic flood, and they still lack Platemail, and have no discount. Likewise, Vikings could get Longbows and either Woadies or Huskies as well. Their ong Bow is FU as well, and has Thumbring, but has about 25% less range than a Brit longbow. Viking Huskarls may have full armor and more hp, but there's no flood effect again. Similar idea for Celts in that their Woadies have +16 hp FU, but they are slower than the Celt version, and therefore poorer.
This building is a bit easier to spam than the Castle of course, costing 450w is still steep however, and production speed is about 10% slower than a regular production speed.