Example: Argonian Backgrounds
My example is based on and builds on the tribes/subraces of Argonians.
Based on existing lore, here are some prime candidates for Argonian Backgrounds...
Agacephs: Most have needle-like faces and vary in color from bright green to orange. Found in inner Argonia near the Hist.
Paatru: Described as "toad-like", they live in the inner part of Argonia.
Sarpa: Described as being "winged", though it's unclear if this is meant literally or not. They live in the interior of Argonia.
Nagas: They are described as having "huge mouths filled with dripping needle-like fangs". Usually seven to eight feet tall, the Nagas appear to be more snake-like than other Argonians. When the other races were attempting to put plantations into Argonia, they generally acted as highway robbers; however, since the other races ceased trying to put plantations in Argonia, they have left the roads and have gone back into the inner swamps
For those who want cosmetic-only changes, this gives the ZOS artists some fun things with which to work. Although Nagas will probably be NPC-only if they are adapted at the size mentioned. Other tribes not mentioned here have no distinction of appearance mentioned, suggesting they have a more standard Argonian appearance and that their differences are largely or entirely cultural (that doesn't disqualify them, but anatomical/physiological variation would be more striking than ethnic distinction for new types of in-game Saxhleel). That still leaves the Agacephs, Paatru, and Sarpa to play around with.
For the actual system proposed, here is one way these backgrounds could be fleshed out (and again, if you think these numbers are too big, picture them smaller):
Agacephs
While all Argonians are connected to the Hist the Agacephs -- distinguished by their brighter coloring, thinner build, and needle-like faces -- live close to its inner core, among the oldest groves. This grants them a greater attunement to the ebb and flow of living energy, making them excellent healers.
New passive tree option:
(*) Increases the Armor value of Light Armor by 10%.
(_) The cost of healing spells is reduced 3%.
(_) The cost of healing spells is reduced 6%.
Single passive option:
The cost of healing spells is reduced 6%.
Paatru
The shorter and squatter Paatru are masters of an ancient and nearly extinct form of shamanism from the dawn of the Argonian peoples. This arcane knowledge grants them greater skill when engaged in the magical arts.
New passive tree option:
(*) Increases Maximum Magicka by 4%.
(_) Increases Spell Critical chance is increased by 4%.
(_) Increased Spell Critical chance is increased by 8%.
Single passive option:
Spell critical chance is increased by 8%.
Sarpa
The Sarpa are known for their swiftness and agility (on land as well as in the water), which makes them ideal candidates for the Shadowscales when open to recruitment. However, living so far into Argonia's interior, they prefer their traditional pursuits such as hunting, to which they are ideally suited.
New passive tree option:
(*) Duration of snares is reduced by 40%.
(_) The cost of stamina-based skills is reduced by 2%.
(_) The cost of stamina-based skills is reduced by 4%.
Single passive option:
The cost of stamina-based skills is reduced by 4%.
Standard Argonian (Saxhleel?)
(Maybe Lawrence Schick could offer a name? Maybe Saxhleel refers to "standard" Argonians and is sometimes used by outsiders (and unifiers among insiders) to refer to all subraces?)
The Saxhleel (or whatever name to use for standard version) are the most well-known of the lizard-folk. They are also the type most associated with the cult of Sithis and their militant wing, the Shadowscales. As a result of their contact with outside races, they developed skills that aid in guerrilla warfare tactics, such as the effective use of stealth.
New passive tree option:
(*) Allows you to enter stealth more quickly and increases speed while stealthed by 10%.
(_) Increases out of combat Stamina and Magicka regeneration by 10% while in stealth.
(_) Increases out of combat Stamina and Magicka regeneration by 20% while in stealth.
Single passive option:
Increases out of combat Stamina and Magicka regeneration by 20% while in stealth.
[Note: The Backgrounds offered here focus on tribes with physical differences but other tribes exist. Other tribes given in the lore may be from the subracial types hinted at for the Backgrounds above, i.e. thin/colorful/needle-nosed, short/squat/toad-like, etc. Perhaps Argonians care more about tribal affiliation than physical appearance. Perhaps not. Maybe the Archeins, for example, who tried to capitalize on the slave trade, included those of the standard/Saxhleel subrace. Maybe the Gee-Rusleel, who are known for their connection to the Hist, tend to be of the same subrace as the Agacephs, hence they both live near/share a deep connection to the Hist. And so on. Lots of room to play around here.]
Thus we have the lore-based Agacepths (healing bonus), Paatru (magicka bonus), Sarpa (stamina bonus), and Saxheel (stealth bonus). Because Argonians are magicka and stamina neutral, the relatively small bonuses do not somehow make a character over-powered.
Other Races
I haven't fleshed out the Backgrounds for other races. I think three or four options would be good. My Argonian example uses four options, so if I were going to work on Backgrounds for other playable races I would likely stick with that.
Here are a few initial sketches of early thoughts on a few other races. If I took the time I could get four for each, but here are the first glimmers of small number of potential Backgrounds:
Redguard
Breton
Orsimer