My suggestions.
Units
Rogue Units
First, bring brack the Privateers of Civ 3. They were a fun unit as they let you cause a little havoc and disrupt your opponents without provoking war. In fact lets expand on that idea. A ground unit called 'Bandits' or 'Brigands' which can be produced by any power but can not be identified to a faction, freely cross borders and attack without provoking war. For fairness purposes they'd naturally have to suck in combat (probally somewhere around a scout), can't capture workers (just scare them off) and they couldn't conquer cities. Both units would make a great mid-game unit for Barbarian states so they can remain somewhat relevant. Perhaps a diplomatic penalty could be incurred if a power produces too many 'rogue units' to help balance out the advatanges of the unit type.
Infiltrators. A Spy-like unit that can cause unrest by staying in an enemy city. Spies lost alot of their usefulness in Civ4, give them a lesser equivalent and then pump Spies up a bit so that overall the option of Espionage is relevant.
Peacekeepers. Either the special unit for a Canadian faction (hey can't blame us for trying) or a unit enabled for the leader of the U.N., basically its pitiful on the offense but gains a massive boost when it gets attacked.
Conventional missiles with less power then the current Nuke that do not provoke the diplomatic resolution. In addition other variants such as chemical warheads (SCUD) could be used as special units for some factions.
Crusaders I'd like to see a religious-based unit for each faction that founds a religion, probally the rough equivalent to the Swordsman unit but with the ability to spread the religion like a Missionary. Maybe just their presence would spread the religion..
New Buildings
Museum
+ Culture, + Chance of producing a Great Artist
Casino
+ Gold Production. Morale Penalty.
New Wonders
The Louvre
Produces a Great Artist. +Culture/every Museum in existence.
Requires 8 Museums.
The Smithsonian
Produces a Great Scientist. +Research/every Museum in existence.
Requires 8 Museums.
New Improvements
Trading Post
Built on the border between two factions. An early improvement which gives a bonus in trade between the two factions.
Radar Station
Provides sight for a large range around it, including out-to-sea and into enemy lands. Gives a bonus for interception with aircraft within its radius.
New Techs
Piracy
PreReq; Sailing
Enables; Privateer unit, Brigand unit, Plunder upgrade (when this unit destroys another unit, gain gold equal to half that units value)
Fanaticism
PreReq; Priesthood
Enables; Crusader unit (if you founded a religion), increases spread of state religion, decreases spread of foreign religions.
Paranoia
PreReq; Nationhood, Radio
Enables; Infiltrator unit, Can build Radar Station.
New Factions
Pirate Nations
Leader: Henry Morgan and/or Blackbeard
Special Unit; A Frigate variant or an early gunpowder unit (Buccaneers?).
Starting Tech; Piracy (see above), Sailing
A faction based around Naval Warfare, similar to the current Vikings. All coastal Pirate cities would be considered connected for trade purposes even if they normally wouldn't be. All Pirate Units would gain the Plunder upgrade (see above). Horrible Diplomatic relations penalties and extremely high maintainenece.
Cities: Tortuga, Port Royale, Trinidad, Vera Cruz etc
Canada
Leader: John A. MacDonald
Special Unit: Peacekeeper (see above)
A faction based heavily on diplomacy, gains a natural bonus to diplomatic relations. Unit maintainence is high. War weariness sets in early and is more pronounced then other factions.
Cities: Ottawa, Toronto, Montreal, Vancover, Calgary etc
New Mechanics
Raid
Instead of the option of Conquer or Destroy, lets add a Raid option. Basically your troops plunder the city like mad, destroying most buildings and gaining the equivalent value of the buildings they destroy. This would be useful for factions who are too far away to hold a city but also don't want to destroy the city.
Anyways just some ideas I cooked up.. probally all been mentioned before.
Units
Rogue Units
First, bring brack the Privateers of Civ 3. They were a fun unit as they let you cause a little havoc and disrupt your opponents without provoking war. In fact lets expand on that idea. A ground unit called 'Bandits' or 'Brigands' which can be produced by any power but can not be identified to a faction, freely cross borders and attack without provoking war. For fairness purposes they'd naturally have to suck in combat (probally somewhere around a scout), can't capture workers (just scare them off) and they couldn't conquer cities. Both units would make a great mid-game unit for Barbarian states so they can remain somewhat relevant. Perhaps a diplomatic penalty could be incurred if a power produces too many 'rogue units' to help balance out the advatanges of the unit type.
Infiltrators. A Spy-like unit that can cause unrest by staying in an enemy city. Spies lost alot of their usefulness in Civ4, give them a lesser equivalent and then pump Spies up a bit so that overall the option of Espionage is relevant.
Peacekeepers. Either the special unit for a Canadian faction (hey can't blame us for trying) or a unit enabled for the leader of the U.N., basically its pitiful on the offense but gains a massive boost when it gets attacked.
Conventional missiles with less power then the current Nuke that do not provoke the diplomatic resolution. In addition other variants such as chemical warheads (SCUD) could be used as special units for some factions.
Crusaders I'd like to see a religious-based unit for each faction that founds a religion, probally the rough equivalent to the Swordsman unit but with the ability to spread the religion like a Missionary. Maybe just their presence would spread the religion..
New Buildings
Museum
+ Culture, + Chance of producing a Great Artist
Casino
+ Gold Production. Morale Penalty.
New Wonders
The Louvre
Produces a Great Artist. +Culture/every Museum in existence.
Requires 8 Museums.
The Smithsonian
Produces a Great Scientist. +Research/every Museum in existence.
Requires 8 Museums.
New Improvements
Trading Post
Built on the border between two factions. An early improvement which gives a bonus in trade between the two factions.
Radar Station
Provides sight for a large range around it, including out-to-sea and into enemy lands. Gives a bonus for interception with aircraft within its radius.
New Techs
Piracy
PreReq; Sailing
Enables; Privateer unit, Brigand unit, Plunder upgrade (when this unit destroys another unit, gain gold equal to half that units value)
Fanaticism
PreReq; Priesthood
Enables; Crusader unit (if you founded a religion), increases spread of state religion, decreases spread of foreign religions.
Paranoia
PreReq; Nationhood, Radio
Enables; Infiltrator unit, Can build Radar Station.
New Factions
Pirate Nations
Leader: Henry Morgan and/or Blackbeard
Special Unit; A Frigate variant or an early gunpowder unit (Buccaneers?).
Starting Tech; Piracy (see above), Sailing
A faction based around Naval Warfare, similar to the current Vikings. All coastal Pirate cities would be considered connected for trade purposes even if they normally wouldn't be. All Pirate Units would gain the Plunder upgrade (see above). Horrible Diplomatic relations penalties and extremely high maintainenece.
Cities: Tortuga, Port Royale, Trinidad, Vera Cruz etc
Canada
Leader: John A. MacDonald
Special Unit: Peacekeeper (see above)
A faction based heavily on diplomacy, gains a natural bonus to diplomatic relations. Unit maintainence is high. War weariness sets in early and is more pronounced then other factions.
Cities: Ottawa, Toronto, Montreal, Vancover, Calgary etc
New Mechanics
Raid
Instead of the option of Conquer or Destroy, lets add a Raid option. Basically your troops plunder the city like mad, destroying most buildings and gaining the equivalent value of the buildings they destroy. This would be useful for factions who are too far away to hold a city but also don't want to destroy the city.
Anyways just some ideas I cooked up.. probally all been mentioned before.