Byzantine Features

The Byzantine-themed update, 1.0.5, was technically first themed update of VIET, although it was not known as such at the time. It included additions to the way factions and claimant wars worked in the ERE as well as 10 Byzantine flavor events, among other minor changes and additions.

Claimants
Byzantine gameplay in VIET does not differ significantly from vanilla except for one major difference - vassals of the Emperor now have the ability to lay claim to the imperial throne. Unlike in vanilla, where you would have to use a chancellor to fabricate a claim, or get one from your parents, in VIET, you can get a claim on the empire through other means.

Currently, there are two ways to get a claim on the throne: once certain requirements are met, you can fire a decision that gives you a weak claim on the throne; alternatively, there is a chance you'll get the claim yourself following a successful battle.

As a result of these changes, ERE claimant factions can and will often support non-dynastic lords to the imperial throne, making the position of Byzantine emperor quite dangerous indeed. In VIET test games, the empire will shift dynasties frequently, although what tends to happen is that the empire will be unstable for a decade or two (or even more) as it constantly shifts between different dynasties, until one dynasty eventually manages to take control and dominate the empire for several decades, possibly close to a century if they're lucky. Things start to get unstable again after a while (if a woman is on the throne, for instance, or there is a quick succession of child rulers) and the process repeats itself. Nevertheless, by the end game, only a few dynasties tend to control the throne. Ultimately, this back and forth between stability and instability provides a progression of history for the Byzantines much closer to that of actual history, unlike in vanilla where the Doukas often sit on the throne forever.

The Turks
While not directly related to the Byzantine features, the Turks have gotten a number of new features that make them more aggressive and more of a threat to the Byzantines. At the 1066 start, the Seljuks can spark off an event chain (taken from Project Balance) where it is highly likely that they will overrun much of Anatolia and form an independent Sultanate of Rum like they did historically. Additionally, the Turks get special mechanics encouraging them to attack the ERE.

Ultimately, even a strong Byzantine Empire will have to be wary of the Turkish threat.

Tips and Strategies
For the player, playing as the Emperor will prove extremely challenging, as you will have to face internal and external threats at once. It is advisable that you constantly monitor your empire for threats and know what your priorities a re. Avoid getting distracted with costly wars or reckless intrigue that could turn your vassals against you - instead, identify vassals who pose possible threats due to their claims to the throne and seek to neutralize them. As a last resort, if fighting against a claimant proves too difficult or costly, always remember that you or another family member can reclaim the throne again one day, even if it is several generations down the line. Meanwhile, always be war yo f the Turkish threat, and always have enough troops in case of a conflict.

If playing with Project Balance, the difficulty as Byzantine Emperor is increased manifold, as you'll also have to deal with the Norman threat and an even more effective Seljuk Empire.

If you're a Byzantine vassal, don't necessarily seize the throne at the earliest opportunity - wait until your liege is distracted with the Turks, for instance, and know ahead of time what other families in the Empire could be a threat to your imperial ambitions. They could either seize the throne before you do, or pose a serious threat to you once you come to power.

Misc.
Unlike later themed updates, the Byzantine one does not include too many minor features and additions for the ERE. It did however include 10 generic flavor events for the Byzantines, as well as a few new minor titles for the ERE.