Actually it's not a new idea, one of the policies in the base game works this way.
It will work the same way as the policy that is in the base game which gives the same effect. I don't remember how it works, will have to check as it's an important balance factor.
haha.. you noticed I haven't ciV yet
[*] A2 Entertainment for Masses: +1 Happiness from every Circus and Colosseum
[*] A3 Social Engineering: +2 Happiness, free policy
[*] B3 Imperialism: +3 Happiness and -20% Gold/Research/Culture in assimilated cities (thanks to Calavente for the idea)
you're welcome.
I'll have some coments : A2 seems more powerfull than A3 (roughly 2 happiness /city if the city is developped. compared to only 1... empire-wide ... unless the 1 is "per city"... in which case it seems ok.
As for "imperialism"... same remark as before : 3empire-wide seems a small gain, as IIRC, your "unhappy /city" is ... 10... and 0.5/pop. so 3 empire-wide is meh.
My earlier proposal was "3happy per annexed city" (so expending is not as costly happy-side), compensed by the -20% in gold/research/culture/(gpp?)... in the same annexed cities.
[*] B2 Slavery +1 Production from Quarry, +1 Gold from Plantation, +10% Production in assimilated cities, can build Slave Market* (now I know how to make buildings buildable only with specific policies, and perhaps I'll add buildings to some other policies as well...)
* Effect of this building is not determined yet. Ideas are welcome

I think it can give some production and cause +1 uphappiness, but I don't know how to do it, unhappiness would be useful for some other buildings as well, like the Tax Collector. Some time ago I tried to assign a negative happiness to a building in XML, but it didn't work. Probably it will require some complicated LUA code, or a DLL change.
I've got some ideas for "slave market" :
+ 1-2-3 gold (fixed income seems better than %... especially if slaves already improve quarry and plantations... you wouldn't have those have both the improved yield and the yield boosted by a %)
risk of "slave revolt" event : "troubles in city for 1-5-10 turns"
enables one of the following :
- allow slaves in city to be "sold" for + production or +gold.
- enable to build "slave stands" (maybe 1 per race you have ?, maybe some slave stands may be unlocked with different techs) (using-up the slave)
(luxury slave stand: +1 culture +1gold, combat slave stand: -1unhappy +1gold, heavy-weight slave stand : +2production (or 1free "citizen specialist"; cannot be assigned to a specialisation (sorry, don't know if ciV has specialists))
another idea would be
-slave market: + FOOD (food %), opens 1 specialist slot of each type, +unhappiness
(reasoning is the following : the civilizations with slavery had most of their "unskilled" work done by slaves, which costed less than free-workers, liberating more people to do specialised work. Athenes could only work as a republic because half (or more) of it's population were slaves, doing grunt work.. thus other 'free" citizen could spend time develloping culture, embassies, artisanry, architecture, military prowess (sparta)...Etc
so while it seems contradictory that a slave market increase possibility of specialists, in reality, by providing a influx of "cheap" labor (even specialised labor), the native people could become more specialised..
in game-play that could either be translated by : more food AND opened specialists slots so you can free some citizen to put them as specialists.... OR giving soem free specialists, without opening the slots..)
my 0.2
(I'll think on that and amend/edit latter as I think on this...)