Civil Wars
Civil Wars, your country reaches a breaking point (could be too much dissent, being at war for 80% of your turns, not enough militia / army, cities are too far from capital etc). You would get a few popup warnings, to try to prevent this and quell rebellion, but once it falls apart there is no turning back -
your faction would split as a royalist and independent - you can choose who to back, then some of the core provinces / provinces far from capital would split up, creating an internal war (with some dynamic pre-generated units depending on what year / strength of faction that has civil war is at)
Re-Emergence of Factions
Also, perhaps after conquering a faction, they have a chance later to re-rise, for example you HRE defeats Denmark early in the game, then in late period you receive a popup saying the ancestors of Denmark have been plotting for years against the HRE oppressors and have finally gained enough support to emerge again (in the same Denmark core province of course) but with a large army at least.
Re-Using Current Faction Graphics
The units for the group civil war group who "splits" from the main faction would use the same unit graphics etc, and similar king / unit names most likely as well. Maybe even the same flag, but a different color to separate them (as they are derived from the main faction) they do not need an entire overhaul in the graphics department like an entirely new faction would.
Also obviously, the faction which re-emerges will use the same graphics it did before they were defeated. These graphics are actually just sitting there unused once they are conquered, so re-emerging and using them would be a good use of this content.
Old-School MTW I
These are two of the features from the original MTW that I really enjoyed, as it gave another political and action dimension to the game (as well as historically many factions did split up or fall and rise again)
I know this would take a lot of work and fine tuning / balancing, as well as may not be a priority right now (as adding more factions etc seems to be the direction TLR is going at the moment, which is great btw) but civil war events and re-emergence of factions would be really cool. Also, I believe at least in the graphical department most of the work is already done. The part of scripting in these events, however, may take some time and skill to do properly.
I leave this as a suggestion for Tornight and also all the TLR fans
-Shatstaffel