Someone do a vegan roleplay, no building pastures or camps allowed. EDIT: and no fishing boats either. Almost forgot.
Heh, that really does sound cool. How about a hippie challenge:
During the entire game you may not build Pastures, Camps, Lumbermills, Mines, Wells, Offshore Platforms, Whaling Boats, Fishing Boats, Workshops, or Forts. Any such improvements you might acquire (through conquest or culture etc.) must be pillaged immediately. You may also not chop any forests or jungles.
You may not build any buildings that give unhealth, be it directly, through power or through resources (even if you can never actually get these resources, such as oil or coal), unless you already have a Recycling Center in that city. You may never build Coal Plants or Nuclear Plants. You may never build Jails (unless you have a UB replacing it), Security Bureaus or Intelligence Agencies.
You may not through trade aquire any animal (that counts Ivory, seafood and Fur too) or mining resources. Should you have accidentally founded one of your cities on any of these resources you must give that city to someone else at the earliest opportunity.
You must do everything you can to keep the peace, give in to every tribute or gift demand as long as it doesn't force you into a civic you aren't allowed to run (see below), never declare war or enter defensive pacts, and end any war that is declared on you as soon as possible through negotiation. You must also try to end wars between other civs, be it through bribery or AP/UN. You may not demand tribute from other civs, but you may ask for gifts. If someone asks you to switch into a civic you are allowed to have (see below) or a different religion you must also comply. The only demands you must not comply with are switching into a forbidden civic, declaring war or cancelling trade agreements with others (you are allowed to pick whichever option you want when asked for the latter).
Excluding Scouts and Explorers, you may not train more military units than you have cities. You are not allowed to capture any cities that didn't at one point belong to you already. You are not allowed to pillage improvements except for the ones you must pillage according to the above rule regarding that. You are not allowed to build any Nukes, or even the Manhattan Project for that matter.
You may not switch out of your initial civics, or any civics really, unless you are switching to the following: Any Government civic but Police State or Despotism (you must switch out of this as soon as possible), any Legal civic but Nationhood or Barbarism (same as Despotism), Emancipation, any Economy civic, any Religion civic but Theocracy. The following civics have the highest priority and must be adopted as soon as possible and may not be switched out of unless you are asked by an AI to switch into another, not forbidden, civic: Universal Suffrage followed by Representation followed by Hereditary Rule (i.e. If you can run US do it, if that's not possible run Representation and if you haven't unlocked that either run HR), Free Speech, Emancipation, Environmentalism, Pacifism or Free Religion (both of which have the same priority, allowing you to choose).
You must beeline techs that unlock civics with the highest priority, though the exact order you may determine yourself. Everything else you must trade or steal (yo information should be free, maaaaaaan) for.
For bonus points play Gandhi of India.
There, that should just about cover it. Now who is masochistic enough togo play a game with those rules?
Oh, I love this game, creates so much "side emotions"... Like - some time ago it was regular stuff to kill Stalin in civ4 because my grandfather was sent to Sibiria because was on "wrong side" during war (both sides were wrong at some point of war). Funny that German hasn't 4th leader... could kill him too (he was the "wrong side") 
