Mongols- The UA sucks, really badly. Something that might work is Horse Resources are increased by 50%
America- Change the buying tiles. Sight is pretty good, but for tiles, say workers improve 50% faster, which is a useful bonus.
Byzaantium- Add "Get Free Prophet at Theology. (im basing this of a Byzantine wonder, the Hagia Sophia)
Denmark- Not much i could think of here, but at least make the free pillaging availible for all units.
Iroquois- Get rid of current UA. Units get Woodsmen Promotion. Farms may be built on forests without chopping them. Longhouse- change it to replaces Granary, gives it +2 production, similar to +2 science, +2 faith, +2 gold etc from other early UB's.
Dutch- Teach the Dutch AI to settle marsh more frequently and to be more willing to trade 1 copy luxes.
France- UA should be more substantial, such as more culture from GW.
Huns- Battering ram replaces Catapult. The UA has a bunch of random stuff, so I would give them 5% against cities.
Japan- Zero leaves. Dojo comes in, replaces Barrckes gives an increase to military production
Ill do more later.