The only problem with the ideas mentioned is that you have limited options for DLC, something that Firaxis loves to release to make more money. Most people would rather play as Carthage than develop a civilization with similiar traits as Carthage. UU's and UA's are a part of why DLC sells.
Since they've never been given any other choice than 'playing as Carthage' in any Civ game, how do you know that's the preference? Especially when you do NOT play as Carthage, but as the programmer's conception of Carthage, which is generally only a single 'snapshot' (Hannibal crossing the Alps) of Carthage.
As for DLCs, there are actually more possibilities for them when you divorce them from specific 'civilizations'. Instead of getting a package that includes 1 - 2 civilizations, they could sell a package that includes a bunch of new UAs or UUs and the paths leading to them.
For example:
DLC: Ancient UUs:
Includes the development path in required Resources, Social Policies, Religious Beliefs, or Starting Position and the artwork and characteristics for the following units:
Theban Sacred Band - a supercharged Hoplite
Thracian Peltast - a cheap alternative to Swordsmen
Khmer Elephant - a supercharged Indian Elephant
Assyrian Siege Tower - an early Siege Machine - no waiting for the Classical Era!
Scythed Chariot - a Chariot Melee unit - deadly against infantry units
Polyreme - Naval Melee unit with increased effect against cities
Pentakonter - an early Naval Scout
Celtic Chariot - another Chariot Melee unit: good in rough country
Dacian Rhomphiamen - melee unit with increased effect against other Melee infantry
Prodromoi - Light Horsemen with lances - deadly against other cavalry
- and this list is just off the top of my head in a few minutes - there are literally thousands of possibilities among all the possible UAs, UBs, and UUs.
Similar 'DLC's could be produced for UBs or UAs in almost all eras - even sets for specific 'scenarios' like a set of World War Two or French Revolutionary Wars UUs.
The basic game might be designed around 3 - 4 Unique traits (units, attributes or buildings) as now, but DLCs could expand that, and the number of potential unique traits for civilizations not normally included in the game: when did we ever see Polish Winged Hussars in a Civ game - now, it's quite possible, along with even more obscure but intriguing units, buildings, wonders, etc.
