OK, it seems this idea has legs, so I've split it out into its own ticket (patch #3843).

Back to this one: the basic nationality mechanism is complete for 2.4.x, and open tickets that remain are about doing more things with it, which will not be in 2.4.x. So I don't see any point keeping this ticket open.
(If your favourite idea in the comments doesn't have its own ticket, do raise a new one.)

> If so, would need some way of asking permission from the
> receiver, which would be tricky UI-wise as pop-ups are already
> problematic, and we don't want donors abusing such pop-ups
> maliciously during a time-critical situation.

What about just sending an event (chatline/messagebox) to receiver at the time when donor tries to add settler. Then the receiver can go to city dialog when ever it suits him/her for accepting new citizens with a button added there.
Problem is what happens to settler meanwhile. Should donor lose it immediately? Permanently (even if receiver declines)?

> About 'underground independence': what should be the death of a player? if there is no city and the last unit of a player is killed? Or if there is no citizen left? Or should all citizens convert to the owner of the city if the last unit is killed?

There is no city and the last unit of a player is killed.

-One question is, if it should be a 'real' ruleset option as it is now or an effect.
I prefer an effect.
-The later would allow to change the result
Buildings (ex. Courthouse and University) and wonders (ex. French Revolution? 1848?).
-Which are good for the game play and should be implemented?
Ruled by foreign oppressor, Ethnic tensions and Underground independence movement.

I would like to include the missing patches in freeciv 2.4 so that the citizens feature is useable (patch #2365, patch #2366 and patch #2367). One question is, if it should be a 'real' ruleset option as it is now or an effect. The later would allow to change the result due to buildings etc.

I will send rebased patches ...

(words to comment #1 and comment #2)

> Wow, this is something that's been on my wish-list for years. :)

The idea is from the greyciv patch from 2004 ([1]; last patch[2]; more discussion [3] and [4] ); but I started from scratch as the code base changed a lot and I want a clear implementation which can be (de)activated by a ruleset option

> The concept of citizen nationality could add some interesting
> dynamics if applied appropriately. Some ideas...

But this has to be discussed carefully; I did implement or plan to implement some of your ideas. Here are my comments:

> -A city with only 'foreign' citizens gets an additional
> unhappy citizen. (Ruled by foreign oppressor)
>
> -A city with mixed 'foreign' and 'domestic' citizens gets two
> additional unhappy citizens. (Ethnic tensions)

> I prefer the way it was done in civ3: citizens of an ethnic
> group at war with the leader start as angry. It makes things
> more challenging to capture a decent size city and not have it
> go into disorder right away.

I do plan a new feeling level after cities which adds additional unhappy / angry people if you are at war with this nation.

> -A city with majority 'foreign' citizens breaks out and
> rejoins its 'original' owner after X turns of civil unrest.
> (Underground independence movement)

OK; I did plan a random element but do it after a time of civil unrest (disorder) is a good idea. The city will go to the nation with the most citizens

> -When a city grows, the nationality of the added citizen is
> that of the majority in the city. (Build settlers and send to
> a conquered city to 'colonize' it)

Here I do add new citizens with the nation of the owner. It is easier for game play. Furthermore, after x turns (ruleset option) one foreign citizen is converted and if you conqueror a city half of the foreign citizens convert to your nation.

I think this will be changed to an effect. Thus, it can be different for different goverments as well as wouders with such an effect are possible.

> -Re-imagining civil war: New player is not created. Instead,
> any city with more than X% foreign population reverts to the
> player of the majority nationality in that city.

OK; this was also my plan (plus the new city counts as conquered; see above)

> -Introducing wonder that gradually 'converts' citizens to your
> 'domestic' nationality (in city or on continent.)

see above; needs this as effect and not as direct ruleset option.

> -Certain governments have effects on nationality; e.g. under
> Communism or Democracy, 'foreign' citizens does not cause
> unhappiness in cities.

needs it as effect (see above)

Links:
[1] http://forum.freeciv.org/viewtopic.php?t=204
[2] http://download.gna.org/freeciv/contrib/modpacks/greyciv/greyciv-0.3/
[3] http://forum.freeciv.org/viewtopic.php?t=206
[4] http://forum.freeciv.org/viewtopic.php?t=6125

Wow, this is something that's been on my wish-list for years. :)

The concept of citizen nationality could add some interesting dynamics if applied appropriately. Some ideas...

-A city with only 'foreign' citizens gets an additional unhappy citizen. (Ruled by foreign oppressor)

-A city with mixed 'foreign' and 'domestic' citizens gets two additional unhappy citizens. (Ethnic tensions)

-A city with majority 'foreign' citizens breaks out and re-joins its 'original' owner after X turns of civil unrest. (Underground independence movement)

-When a city grows, the nationality of the added citizen is that of the majority in the city. (Build settlers and send to a conquered city to 'colonize' it)

-Re-imagining civil war: New player is not created. Instead, any city with more than X% foreign population reverts to the player of the majority nationality in that city.

-Introducing wonder that gradually 'converts' citizens to your 'domestic' nationality (in city or on continent.)

-Certain governments have effects on nationality; e.g. under Communism or Democracy, 'foreign' citizens does not cause unhappiness in cities.