G'day, and thank you! I'm not the lead developer but I can discuss some of the points you've made.

1. We plan to make the gameplay viable as a multicultural nation. How that happens and the systems involved is still being planned. One of *my* plans on paper is a large array of buildings and wonders (my specialty in Imperator modding) that improve wrong-culture, wrong-religion (sub-culture in LotR mod).

2. The beast pops seem to be bugged from your description, that's definitely going to get fixed.

3. Again, the age thing is a bug that is being worked on and fixed.

We've come a long way and there's still a long way to go. As you can probably tell from the world map, there's some parts that are simply bare bones. Thank you again for your post!

Yeah, multicultural nations need to be workable (and I think they are), but there are no plans for cultural assimilation at all as we cannot do it in a case-by-case basis. In other words, I don't mind Dunlendings assimilating to Rohirrim, but doing that will also mean that elves can become dwarves, which we certainly don't want.

Ahh - so that's where the beast pops that people are noticing come from. I'll make a "beast pop/char" killer annual maintenance event for the next version.

I'll look into the aging thing - it's possible that I didn't catch all the events that give diseases and prevent them from being given to elves and non-old dwarves and dunedain.

In my humble opinion an "ethnic cleansing" law(so political power cost+stability hit) could do the trick for flavour, enabling assimilation but also giving maluses to public order, could be enabled by default in evil races.
In laws you can put every kind of loyalty and public order malus, the templates are almost already there.

In my game there are elves with arthritis, kind of funny.

The multicultural problem is not really public order but that you get no discipline bonus. A possible solution may be to assign a bonus to every race(I've noticed that higher discipline bonus is also a lower growth rate) and weight them according to the composition of the civilization.

for example: elfpop%*elf_modifier+dwarfpop%*dwarf_modifier+Dunlendingpop%*Dunlending_modifier

We'd prefer to stay away from such horrors as what ethnic cleansing is defined as: https://en.wikipedia.org/wiki/Ethnic_cleansing . We certainly won't be using that word. =p

I don't mind that there will be Dunlendings in Rohan. We're still working on our new systems, bug fixing them and adding new game play features. And we are adding new buildings to decrease the negatives of having a multicultural and multi-species nation. I've brainstormed a new building called the Inn that increases happiness for wrong-group cultures.

I like your idea of re-adding the discipline bonus. You may be interested in this titbit of lore:

In Tolkien's drafts for the Muster of Dunharrow there are Dunlendings that join forces with the Rohirrim. Aragorn says to Theoden that many Rohirrim have answered the call: "'And with them have come also strange folk that are not of Rohan. For in some manner, the rumour of war seems to have gone far abroad long days ago, and men in distant countries have heard the word go forth that all who hate Mordor should come to Edoras or Minas Tirith. There are tall warriors of Dunland, some that fought against you, and some that never listened to Saruman, hating the Orcs far more than the Rohir! There are even Woodmen from the borders of Mirkwood, and wanderers of the empty lands. Last and fewest, but to me not least, there have come seven Rangers out of the North, my own folk, remnant of Elendil's race: they have sought me here.'"

In a human based setting you're right, in a fantasy setting I'm pretty sure that's what orcs do. Just butcher and enslave anyone, you would not really see an orc trying to get along with elves, on the other hand when you conquer an orc territory the population becomes an infantry unit.
That one is an amazing mechanic that makes sure you have no orcs as pops.

Yep, it's a matter of implementing gameplay systems.

I played it some more and I feel pretty confident to have a suggestion for a mechanic that may improve it.

This mechanic is best suited as a law, to give player the choice, with two possibility.

first one is a drive to multiculturalism(I would say default option for "good" factions), gives a little bonus to foreign culture groups, malus to your own and lets your faction discipline be weighted over your race composition in the realm. For example and motivation, if you conquer elves or free them from Melkor they will fight by your side, your armies the bonus to discipline will take into account not only the flat percentage of your main race but also the fraction of the other ones.

Second one is purge, enabled by default in evil factions, giving massive malus to non accepted cultures(to imply resistance and rebellion to the invader) but also enables you to cultural assimilating.
The explanation would be simply that the other races are seen as outcasts and slaves their population slowly dwindling and dying. Gondorian in Mordor were killed, others corrupted.
It would let you form an omogeneous raced kingdom on the long term but also giving you rebellions from the resisting population.

It would let you to roleplay as a corrupted elf or dwarven with no respect to the other races, if your faction changes from one to another you could drop the equilibrium stability seeing it as a betrayal to the old ways

Originally posted by Snake_In_The_Eye:

I played it some more and I feel pretty confident to have a suggestion for a mechanic that may improve it.

This mechanic is best suited as a law, to give player the choice, with two possibility.

first one is a drive to multiculturalism(I would say default option for "good" factions), gives a little bonus to foreign culture groups, malus to your own and lets your faction discipline be weighted over your race composition in the realm. For example and motivation, if you conquer elves or free them from Melkor they will fight by your side, your armies the bonus to discipline will take into account not only the flat percentage of your main race but also the fraction of the other ones.

Second one is purge, enabled by default in evil factions, giving massive malus to non accepted cultures(to imply resistance and rebellion to the invader) but also enables you to cultural assimilating.
The explanation would be simply that the other races are seen as outcasts and slaves their population slowly dwindling and dying. Gondorian in Mordor were killed, others corrupted.
It would let you form an omogeneous raced kingdom on the long term but also giving you rebellions from the resisting population.

It would let you to roleplay as a corrupted elf or dwarven with no respect to the other races, if your faction changes from one to another you could drop the equilibrium stability seeing it as a betrayal to the old ways

i actually really like what you are suggesting i hope the mod devs add this to the game.