1. Liu Bei / Liu Shan rework. Liu Bei definitely needs an important re-work down the line to stay on par with the new power creep in Cao Cao's faction. I think his mechanic could involve exterminating traitors of the Han. He could have the opposite mechanic of Dong Zhuo where instead of intimidation he has a righteousness meter. Perhaps he could also have a mechanic were he gains some mechanics from other factions such as if he eliminates Liu Zhang he gets a weaker version of the Liu Yan/ Liu Zhang inheritance mechanic so he can try and develop Liu Shan.
2. Sun Quan. I feel that Sun Quan should have a governing mechanic where he obtains additional missions from trusted advisors. Although this might be a little too similar to the new court system. His mechanic could also have something to do with re-developing/ re-building the south.
3. Ma Chao. Ma Chao/ Ma Teng do need a new mechanic. For Ma Chao I think his mechanic should have something to do with raiding and using elite cavalry units. Perhaps he could also have a mechanic were you get bonuses for exterminating rival characters within rival factions. Perhaps he can obtain rivalries with people more easily too.
4. Guan Yu. If we get Guan Yu as a faction or not will depend on if there is a chapter pack between ChiBi and a Three kingdoms. His mechanic should highlight his arrogance were you get bonuses for diminishing your relations with Wei and Wu and trying to aggressively expand. His mechanic can be based on achieving certain things in battle.
5. Sima Yi/ Jin. I think Sima Yi if made a faction should get multiple mechanics. He could have special formations only available to him in battle. For a main mechanic though he should have some kind of meter that goes up by gaining victories for Wei but if you win too much or grow in power Cao Zhen / Cao Rui etc will initially cut your supplies and eventually declare you as an enemy if you get too strong. You should have actions were you can relinquish military command to a general that Cao Rui decides, reduce the amount of troops you have, or return to court to help rebalance the meter.
6. Zhao Ang or Wei Kang as faction leader. Zhao Ang is a faction that I would really be excited about because it would also give more flavor to a Ma Chao campaign, as he and his wife resisted Ma Chao. His Wife Wang Yi, is the female character that I would most want to see as a unique character in the game since she was one of the few women that actually fought. For mechanics perhaps his armies could build special fortifications and get a special stance instead of fortify where they would get a much more substantial fort. Overall his mechanics would make sense to be themed around military defense. Since the new emperor mechanics were added, perhaps his mechanic could synergize in some ways were he gets extra supplies and bonuses for defending against factions declared as enemies of the Han.
7. Zhang Lu. Zhang Lu is a faction that I really would like to see since he would add really nice flavor to the just added Liu Yan/ Liu Zhang's campaigns as well. Zhang Lu would have multiple start dates and would provide a nice narrative for when Liu Zhang invited Liu Bei into Shu to defend against Zhang Lu. Serious trivia in some his videos mentioned that his mechanics could involve assimilating yellow turbans which would be very interesting. This could also serve as an opportunity to fix a few things that the community has been asking for with yellow turbans. Such as Han factions being able to recruit yellow turbans (the way they can recruit nanman officers), Yellow turbans having some brothership hierarchy in place of a family tree, able to spawn more generic characters late game as they tend to run out of characters, an alternative mechanic to spying etc.
8. Zhang Xiu. He is very interesting in my opinion and could be added into a 190 date as well as a Dong Zhuo vassal. His mechanic could be involved around being a vassal and getting bonuses for furthering your vassal masters goals and then transition to a different mechanic late game where you try to usurp your vassal master.
9. Han Sui. Him I definitely expect, his mechanic could involve assembling Qiang and other ethnic tribes where he would get their units. He also could have a mechanic were his characters that show exemplary military prowess can become one of his "eight knights" as his top eight generals where known as han Sui's eight knights.
10.Ze Rong. I could see him being a 190 FLC as he died in 195. The main reason I chose him is because he was a buddhist leader and built buddihst temples. His start would also be interesting because he would start south of Tao Qian, in the south east corer of the map where we don't have any starting warlord currently.
His mechanics could involve converting characters to Buddhism and giving them a special buddhist trait. Also he could have a mechanic which encourages you to backstab allies and steal their territory. (Since he did backstab people historically like Liu Yao) He could also get unique buddhist convert units or warrior monks maybe.