Some notes about current progress:
1. Seems it's not possible to add new attitude levels to diplomacy without SDK changes - so I can't add new attitudes.
2. I've looked through python file that defines the diplomacy window. Looks like that the all those functions depend on diploEvents which are defined in the SDK files and thus I can't really make any changes to the system.
3. New route types can not be added without SDK changes...
4. I won't be adding new religions - this was the decision I made already when I was overhauling the religion system (was it RoM 2.0?). It's not easy task to add them and more of them would just unbalance certain things.
5. "Religion civics" -system would require massive changes to the game engine (SDK). First of all one would have to make new system for religions that works like normal civics. You might wonder why? If normal civic system was used for religious civic -system there would be numerous problems - first of xml files: all normal and religous civics would have to be defined to the same file and this leads to problem with AI players as they would see all of those civics as normal civics and would change both at the same time. Second thing is modifiers - there's too few modifiers just for religion specific options and few of them in certain combinations are currently bugged (also bugged in normal BtS but not used in that way). Third thing is that the religion screen would have to be made from scratch to support this kind of new system - even if some coder managed to make the required SDK changes for new xml files, new schemas, all new civic modifiers, then some expert python programmer would have to make new religion python screen. Fourth is the AI players - this kind of change would require SDK changes to AI so that they would know how to use this system and that won't be easy task. Sixth would be game balance - having 50+ new "civics" that affect everything would be pain to balance and the game engine in RoM is already reaching its limits. So overal this kind of thing won't be happening in v2.6 or in any future version.
6. New leaders will go into extra civ addon pack. Currently about 10 have been added. I can add those which can be found from these forums in modular format ie. all the needed xml files are included with pedia and diplomacy texts (even with those it still takes some time to convert them to RoM settings).
7. Traits - For v2.6 I'll add only positive traits. I might consider those negative ones in future versions.
8. Mod component updates: I've found one bug in random event system from Revolution mod and it seems it was lot bigger problem - this bug has been fixed now in Better BtS AI 0.5 because the bug required SDK changes and eventually it will go into Revolution and RevolutionDCM mods.... so once those mods are updated I'll merge them again to RoM. BUG mod will be updated too in near future and that version should include a new fix to no interface problem (which was found in RoM) in some vista computers.
9. Promotion system - seems I've found some limits from it (175+ promotions) and the system goes goofy when more promotions are added. So I may have to discard some planned promotions unless I can find out what's causing the bug in that system.
10. Diseases: I most likely will add more of them through random events and not like in Euribus World Religion mod (RoM's religion system is based on his mod so I've had that mod for like 2 years now
). His mod was made for Warlords and it might be difficult to convert his disease system to BtS.
11. Scoreboard: the current system in RoM is from BUG mod (MainInterface.py) and it's completely different from Revolution mod's version which had that fancy divider that changes the displayed civs when clicked. So merging those is not "possible", the whole code section would have to be written again to support both BUG and Revolution mod scoreboard features.
Now I'm forgetting something... I'll have to post about them some other day then.
edit: 12. I've been playing Total War: Medieval II (and earlier Total War: Rome). It has few features that interests me and I might try to add couple things to RoM. First would be the overhaul to King Richard's Crusade wonder and I would change it to Project which would then enable Temple Church / Templar Knight's Headquarters national wonder for all players and if possible only those players who have the same state religion as the player who completed the project could build that national wonder ie. join the crusades. With Temple Church you would then get Crusader units every few turns. In Medieval II priest units can fight against other religion missionaries (Heretics) to prevent them from spreading "wrong" religions to your nation - what if this was possible in Civ too?