Tribal Holdings
Tribal holdings (though I don't know if that will be their actual in game names) sound like they will be much like castles, bishoprics and cities, although Doomdark also put trade posts in that list, which leads me to believe they might be an over-holding of some sort (to prevent the awkwardness of having one holding change to another later in the game). Essentially tribal holdings will represent lands that aren't properly fortified yet, more owned by virtue of people living there than by people actually building towns etc.
An interesting aspect of tribal holdings, is that vassals who are 'tribal' (presumably a new title equivalent to count, based on your holding type) don't provide levies in the way that a feudal vassal does. Instead, a tribal vassal must be called to arms, like an ally. In this way, your vassal management becomes much more important, and vassal ties are much looser. A king can only gain power if he is respected enough by his vassals, even more so than currently, and a vassal maintains full control of his own armies, making war much more scattered.
Elective Gavelkind
Another interesting new feature, designed for the breaking up of blobs: elective gavelkind. Empires being too blobby has always been a big concern, and even more so with the introduction of a start date that may very well include the entirety of Francia as one huge empire. At least one response to this, which also adds to the susceptibility of tribal civilisations falling apart without a leader, is elective gavelkind.
From what I can tell, elective gavelkind is much like what it sounds: it's a cross between tanistry and gavelkind. The new ruler must be selected from your dynasty, but I believe that other dynasty members will also get titles, as gavelkind suggests. In addition, upon succession, some vassals may be given the option to become independent, no war involved, much like the decadence mechanics are supposed to work.
The exact nature of this hasn't been settled on yet, since they're still testing it out, but it may be based on the power of the vassal relative to liege, the opinion of the vassal, or perhaps even a choice that everyone is given regardless, and then acted on appropriately. In any case, this serves to make early empires more likely to break apart, and perhaps constantly try and put themselves back together, in a way that current succession laws don't.
My only hesitation with this is that it will probably still be too easy to get a big mid-late game empire going. Once elective gavelkind is gone, I'm not sure what will be used to try and break apart large empires that have formed under primogeniture, or even normal elective. So, while this should solve the early Karling problem, I don't think it will do anything for the HRE, or for later game blobs. We shall see however: blobbing is clearly something in the forefront of the minds at Paradox, so hopefully this will also be settled. One possible solution is:
Vassal Limit
A new limit, much like the current demesne limit, will be imposed upon rulers with the introduction of the Charlemagne DLC. This limit does exactly what it suggests: it imposes a soft-cap on the number of independent vassals you can have in your realm before you start taking penalties. To offset this, you will be encouraged to hand out more duchy titles, and perhaps even kingdoms, as your empire becomes too large for you to manage each vassal yourself.
This is a great idea, and as unfortunate as it may be to have to impose rules like this, rather than providing encouragement for playing the game in a certain way, I think it fits in very well with the current demesne limit. It makes sense that if you can only manage so many holdings yourself, you can logically only manage so many vassals as well before you become stretched thin. This is why vassals exist in the first place.
In addition, this adds a new balancing factor to crown law. As crown law gets higher, and the monarch begins to exercise more direct control over his vassals, the vassal limit will decrease, so that more titles need to be handed out. This means that going up to absolute crown law will not only make your vassals like you less, but also encourage you to give those vassals more power as well, to help maintain the laws you impose.
As said before, I very much hope this will be the way to make empires crumble. With low crown laws, empires can be sprawling, but somewhat weak, with fewer levies to help defend against outside threats and even factions, and less control over their vassals. As crown law increases, larger vassals will come into play, curbing the power of monarchs who they don't like, making factions more likely to spawn. Ultimately, this may see more independence factions firing and being successful. Fingers crossed.
Seasons
Finally, seasonal changes, similar to EU4. I think everyone has been expecting something like this for a while, and I guess that Paradox decided to just throw it in there now. Seasons should have an effect upon attrition in provinces, making war in winter a more dangerous affair, and there was a hint that weather might effect combat, perhaps by changing terrain. Whether this will just affect military educations (fights better in snowy conditions) or whether there will be a weather effect on combat I'm not sure, but in either case this is a change I don't have much to say on other than it's there.
Overall, I think that might be all the information I got out of developers. I didn't have my journalism hat on at the time, so I didn't chase up nearly as many questions as I should have done, but hopefully that puts some fears to rest that people may have had. I know that I started off very disappointed in the idea of a further timeline extension, but having discussed with the devs, this sounds much more thought out than I gave them credit for. Here's hoping that everything goes as well as it does in my imagination.