I would agree with making juggernauts a faction trait, especially if Slave Lords gets changed to allow you to actually train slaves when you have the trait rather than slaves being a bonus from somewhere else (I think a hidden bonus from being Quendar).
Speaking of racial traits, what do people think of those?
Overview (I will put the special 'train a unit' abilities in with the racial, since I don't know of a better place for them, and since custom factions with that race also get the special units)
Resoln - Wraith Blood: -1 health per level, heals three health on killing a unit
Umber - Urxen Blood: +2 Attack when in an army of 5 or more units
Yithril - Trog Blood: +20 Weight Capacity, +1 Attack, Train Juggernauts
Kraxis - Krax Blood: +10 Defense when below 50% health, grants units the 'Fortify' tactical ability (+10 Accuracy, +20 dodge in Fortified tile)
Magnar - Quendar Blood: +50% Fire Resistance, -50% Cold Resistance, Train Slaves (weaker-than-normal troops who have no wages)
Tarth - Tarth Blood: +3 Attack and Initiative when in an army of 3 or fewer units
Altar - Altarian Blood: +10% Experience, Train Henchmen (trainable champion-type units).
Gilden - Ironeer Blood: +1 Health per level, +30 spell resistance, +50% to mana cost of tactical spells, Train Golems
New Pariden - Amarian Blood: +1 initiative per Air Shard, +1 Spell Mastery per Fire Shard, +1 Spell Resistance per Water Shard, and +1 Health in combat per Earth Shard, -1 Health per level
Capitar - Mancer Blood: +1 Accuracy per level, ability to build roads.
My thoughts:
Wraith Blood is terrible for trained units and high-level champions, and okay for low-level champions. Why? Trained units only heal three points, which is usually negligible compared to their health pool, and if it isn't negligible compared to the damage they've taken then they didn't need the healing. High-level heroes get most of their health from leveling up, so this essentially halves the health of high level Wraith champions and reduces by about one-third the health of mid-level champions. For low-level champions it isn't bad, because most of their health is from their base health and three points of healing per killed enemy actually means something at this point.
Urxen Blood isn't bad, but it doesn't seem great either. It looks like the bonus translates to an extra one or two maximum damage when looking at late-game units, which isn't really all that much.
Trog Blood doesn't have any penalties, and lets me drop the Strength trait for something more useful in unit design, or lets me equip heavier armor earlier on my sovereign and any similar-race champions. It also means that I am less likely to have encumbrance penalties, so my units should have higher initiative than identical units (i.e., same traits and same equipment) of other races. I don't think that there is anything bad about this trait, and the +1 attack at all times is more useful than the +2 bonus Urxen get in armies of five or more units, especially since I can make use of the extra attack from the start of the game. Also, you get Juggernauts for this? You need more bonuses to go with this trait? Really?
Krax Blood grants a very nice tactical ability which makes trained units much more useful as meatshields, and when you add in the +10 defense when they're at less than 50% health it just gets better. Not perhaps the best trait but far from the worst.
Quendar Blood would be worse if there were more things in the game that actually did cold damage. As things stand currently, it seems like it's more of a bonus than a penalty, but my view on that would change if I were to find monsters that deal cold damage or an AI that makes frequent use of ice weapons or water magic's damage spells. Being able to train slaves makes this better, but that really feels like it should be attached to the Slave Lords faction trait rather than Quendar Blood.
Tarth Blood seems alright for now, but the game seems to be punishing the use of small armies more and more with each patch, so I don't know how good Tarth Blood will look in the future. If small armies continue to be discouraged by the game, perhaps Master Scouts should be re-worked so that it only applies for armies of the same size that Tarth Blood works for (or, at the very least, lose the monster invisibility when you add a fourth unit). This is good early game, and for when you just can't afford to have big armies, but the bonus is something that I'd forget about if I could afford a large enough army that the bonus no longer applied.
Altarian Blood is good, because it accelerates the rate at which your forces get better. It's bad because it doesn't do anything else (except maybe allow you to train henchmen - I noticed when I made a Quendar Empire with the Heroic faction trait I could not train Henchmen, but I haven't tested this to see if only the race of Men can train henchmen). I'd say that this is one of the better faction traits.
Ironeer Blood is also a good trait - you can get non-Ironeer champions to be your casters and I believe that you've just dodged the only penalty this has. It's great for making tanky troops and champions, and the spell resistance bonus is the largest single boost to spell resistance that I know of in the game. With it also having the ability to train Golems, I'd say that this racial trait is a strong contender for the best.
Amarian Blood has the same weakness in health that Wraith Blood has, and is extremely depended on the number and type of shards available to you, so it can be variable. If you have a whole bunch of Air Shards, then it's great to be Amarian. Having Earth Shards can partially (or, with champions at sufficiently low levels, fully) make up for the lesser health bonus on leveling, but the small boosts to spell mastery and spell resistance from having Fire or Water Shards don't really seem worthwhile, unless you have a lot of shards. Overall, I'd say this is more or less neutral, but it can be very good if you have lots of shards (especially lots of Air Shards, perhaps lots of Earth Shards), or very bad if you don't have many shards (or have mostly Fire when you have sufficiently high level casters that the extra spell mastery isn't that useful, or mostly Water Shards when you aren't facing much that makes use of offensive spells).
Mancer Blood is kind of worthless. The only time that I really need more accuracy is when my units are low-level, and this only provides noticeably more accuracy when my units are high-level. I'd sooner have something else. Also, this doesn't come with any other bonuses, so it's kind of weak. It at least doesn't have any penalties.
So: Good Neutral Bad
Trog Quendar Amarian (with exceptions for certain circumstances - like lots of Air shards)
Ironeer Tarth Wraith
Krax Urxen
Mancer
Altarian