I love the idea that this is a totally untamed and dangerous world where civilisation is going to have a really rough time of it. And I really like the idea that you're too busy up to your neck in barbarian corpses in the early game to even think about some cheesy warrior rush on your neighbouring civs.
The only problem is it seems that, for the most part, reckless expansion seems to actually be the best way to combat the barbarians, rather than a horribly risky and probably misguided idea. You've suddenly fairly permanently reclaimed a whole area from orc etc appearance without really needing to fight for it. And because the very food-rich farms make settlers (and road-building workers) effectively very cheap, it seems to promote city spam (which is exactly what the AI seems to do). And of course, sticking warriors etc on hills around your civ is a much better defense (by magically stopping barb appearance) than actually having them within your borders. I know this is a holdover from vanilla, it just doesn't seem to work as well here, and it means that barbs very soon disappear as an actual problem, unless you're unlucky enough to be near Orthus or something. I play on normal size though, so I imagine the dynamics are fairly different on larger game sizes.
I reckon the ruins and barrows are an absolutely inspired idea, though. I'd love to see them expanded to account for the majority of normal barbarians (orc camp, bear cave etc or something) so that you really do have to actually fight for the territory. I'd also love to see them all start with a defender too, maybe spawn another before starting to attack, and maybe a slower rate of new troop spawn (and less tendency to beeline for cities maybe). So you have to mount a serious barb-clearing assault to clear an area for expansion. Maybe give some reward for clearing them (like a goody hut). Some actual barb cities that exist from the start would be cool too.
So barbs stay a force for longer, are more of a hindrance to actual expansion, and you can maybe even get some of those cool carnival bonuses before they all disappear!
It would help give the early game more of a Master of Magic/HoMM sort of vibe.
For the moment though, I've found that running a pangaea map with "cold" climate is great for leaving big tundra/ice areas at the poles that continue to spawn barbs for much longer (and it fits the theme of the mod, too).