I believe the Romans UB is a forum.
That sounds like an interesting value to start playtesting. I was thinking it might cost even more hammers, the hammer cost of 3 temples. The value of culture for hammers is quite low for your first few, but once you're halfway to the maximum, each building gives the culture value of 3 temples (after that, it becomes an even better value). If you make the cost high enough, the military production modifier isn't impotant, since building these things in lots of cities means foresaking a lot of potential military production.
Another similar idea I had that might be harder to implement (I've not done any real modding) would be to alter the cultural victory condition for the Grigori so that they need to have more than 3 cities reach legendary culture for the cultural win (6, 9, or 12 depending on map size). Then, instead of the above building, have several different culture boosting buildings and effects for the Grigori. A Grigori player following a builders path would become a cultural powerhouse; border cities would peacefully 'flip' as they decide to join the Grigori cause. Of course, this would aggravate other civs who would look to war.
Perhaps in this case some of the buildings could increase their culture output (or effects) the longer they've been built, to emphasize building early for a cultural victory, rather than militarily rushing a large empire before switching over.
Balancing it would be hard, however.
I thought about making the hammer value as expensive as three or more temples, but it occured to me that you're already investing more hammers than three temples for your victory cities. You're investing 200 hammers for a single forum, yes, but you may be doing that in 10 cities, which is a total of 2000 hammers for the effect of just three temples in the only three cities that matter. A single temple of any type costs 120, which means three temples in your three victory cities would only cost 3*3*120 = 1080 hammers. Already you're paying nearly twice the hammers empire-wide for the same benefit, and 200 hammers is no small amount for your less-productive cities to cough up, whereas you can pick and choose your culture cities to get good production/food balance. If each Forum cost the amount of 3 temples, that would be 360*10=3600 hammers empire-wide for the effect of just three temples, a factor of 3 for the same benefit in the only cities that matter. Add in the fact that the Grigori are
still unable to get any extra temples, and I think it's risky to do it any more.
To be honest, unless culture-trade routes pans out (ideally unique to the Grigori), it might need to be stronger. Anyone can research all the religions easily enough, if they work towards it.
This isnt supposed to be balanced for the satus quo to give the grigori an alternative, we are trying to give them bonuses for a culture victory. I think the stated cost is high enough.
It would also be nice to have some sort of mechanic to discourage militia but encourage adventurers. I would propose a Militia unit of sorts, that can be recruited for cheap, like 10 gold, but costs 2 gold per turn, and cannot leave borders. Perhaps if it needs to be limited, have it reduce the pop by 1 but NO draft anger or hurry anger. An alternative would be to disallow militia from using metal weapon promos.
I imagine the Grigori army to be much like Colonial 1760s and 1770s ... only without the presence of the British army for the most part. Mainly relying on adventurers to rally a militia in times of need, and going it alone at other times, or in small groups.
MagisterCultuums militia-desertion event might work well if it was tied strictly to a Grigori-only Militia unit. Have militia units be not-quite-as-strong but much cheaper in cultural boundaries than their comparative normal equivalents. If they go outside their boundaries, though, they become significantly more expensive and open to MC's event.
A problem might be how to implement it as opposed to regular units, without making it impossible to expand early even in cases in which a lone adventurer isn't enough, and a militia-warrior would be unreliable.
Thought: How about Militia replace tier two units like swordsmen and archers? Capable of being built even without a training yard/archery range (though much faster/somewhat stronger if one is present), militia start with a Weak promotion that goes away after X battles (possibly changed by if there was a construction building or not). You can likely pump out enough to defend your territory where they're still cheap, but it's liable to be too expensive to construct the sort of stack of doom to attack your neighbors, since they'd have to be even larger with even higher costs.
Tier-3 units and above (Longbowmen, Dragon Slayers, etc.) don't suffer the militia weakness, being better trained, motivated, and disciplined to not suffer such bouts of homesickness. Militia could also be promoted, of course.
Units that shouldn't suffer militia woes would include Recon units (they know/sign up to go exploring), horse riders (specialists who require more training/dedication than peasant militia), and mages (a different breed altogether).
Alternatively, a Grigori-only promotion/promotion line for units similar to above. The auto-gained 'Militia' promotion acts like a Weak promotion and covers the militia cost increase, as well as serving as the tag to verify whether MC's milita event occurs or not. Ideally the militia promotion would go away/improve with enough battles, as a village militia matures into a peer fighting force.
You might even make it a promotion line, like with the Giant Kin promotions. Raw Militia->Seasoned Militia->Veteran Militia->Elite/Legendary Militia. Raw and seasoned are still weaker and more expensive and a veteran is roughly equivalent (maybe marginally stronger), while an Elite Militia is a reward for shepherding such a unit for so long. Possibly get some special promotion, or a re-usable Recruit ability that makes more militia, because they're just that inspiring.
Thoughts?
Another possibility would be a special building, which slightly lessens the yields of great people, although makes it impossible for anything but an adventurer to be created. Maybe instead it should lessen the yields of regular specialists? like -1 science, -1 gold, -1 culture, or -1 hammers. It would hurt someone using and abusing a specialist economy, but for people desperate for adventurers- i'm sure they are just in it for the GPP points for more heroes.
I don't like. You'd need some serious tradeoff to harming specialists like that; I don't know about you, but I don't just use Engineers for wonders, but to boost production in a poor city. I treasure sages to extend my lead in the tech race. Bards are sometimes critical for opening my borders/competing with other neighbor's culture.
The Cassiel anti-specialist argument has always seemed weak in my mind: Cassiel's ideals are admitted to be regularly disappointed by his subjects, and concentration and specialization have always been the result of more liberal/open-opportunity societies that he leads. He might dislike it personally, but the Grigori themselves would certainly welcome their ability to advance themselves via their strengths.
Like I said, it would need a serious trade off to be justified. If you're interested more in increasing the number of adventurers coming out, the better option would probably be increase the number of ways to get Adventurer GPP (like the Philosopher specialist, more UB), rather than decrease the output of other specialists.