I’ve always wanted to run a Dark Sun game involving character trees involving several different regions of Athas. Each player gets multiple-characters and I run essentially, several different campaigns of Dark Sun, at the same time. For instance, the players would all be from Ur-Draxa one game: another all from the City-State of Tyr and another from, say, the Ringing Mountains or beyond the Tyr-Region. Each game would be different from a gladiatorial game(Tyr), to a politics game involving nobles (Ur-Draxa), to a wilderness survival game (Ringing Mountains). Or one based on Preservers or Clerics. Also, each player would have character that were slaves, others that are freemen and or nobles! I would need to design the game in which the events from one side of the region can effect the game on the other side.
I’ve not used character trees like that before, they’ve always been more along the lines of replacement party members for a single group. Maybe it’s just the way I’ve run the games, but I have had multiple groups in the same campaign before and eventually one group may hear about the exploits of another but until they really some pretty high levels (just before epic or into epic) a single party didn’t make a difference that would be felt across tablelands.
In the scenarios you laid out there, I could see a gladiator game being talked about outside of Tyr, but remember travel between cities is pretty rare. Traders do so, adventurers do so, but most city dwellers rarely leave. Even nobles would tend to stick within the “borders” controlled by the city state. Ur-Draxa was even more cut off than normal city states.
For a Preserver/Cleric campaign, I currently have a “Avangion Council” made up of a few low level ones scouting out preserver/psions to expand their ranks to the point where they can challenge the sorcerer monarchs. They have to move very carefully though since the SMs can swat them like flies. It’s taken them a long time to get to that point. They’ve been creating small havens in mountain valleys to train up additional preservers.
There’s also a “Conclave of Five”, high level clerics of Earth, Water, Fire, Air, and Rain that are working together to return the elemental balance. Each of them is in the process of becoming Elemental Lords but after a pretty disastrous discovery of their stronghold by Borys they fled into the elemental planes and haven’t regrouped yet.
A druid circle may also be of interest since changing the world seems to be the goal. I have a group that hunted down the ancient druids from before the Eradication, learned a decent amount of history, and started trying to organize the druids, under the cover of a trading house.