While tsar is theoretically equivalent to emperor, there was no king equivalent in Eastern Europe, while tsars had more in common with the kings in the west, rather than with the HREmperor. If the example of Kaloyan is sufficient, the distinction was not that obvious for the rulers of the time also (although in that case, he had quite obvious reasons not to see difference
). I would be satisfied with imperial titles be reserved for HRE and Byz as in CKI, as I do not think it realistic to create other imperial tiltes during the timeframe.
The main difficulty would be to create a balanced event/decision chain for the change (if it is a simple relation-based decision it is meaningless). Still there is quite some time to CKII release and hopefully PE has though out some resolution to the problem.
Sticking too much to cold mechanics like tier system or dukes cannot be vassals of dukes, was on of the bane's of CKI (Jihading sheiks was another one
).Lack of flexibility of the title system will make things bland and predictable. In CKI you could inherit enough duchies and recreate a kingdom, and your liege wouldn't mind at all, Your neighbours wouldn't mind. The powers that be will be (pope and the emperors) silent. When a ruler dies he is succeeded by a very specific heir and none of the other heirs will say “but he is actually a bastard” or “but the old king asked for me to take his place on his deathbed”. And that is the main thing that should be added in CKII fluidity, change, semantics of such matters. Creating a new independent kingdom was shattering the medieval and feudal cosmology, holding a title and actually being recognised as such in a world governed by tradition, symbolism and semantics are two very different things and that and similar stuff should be represented in the game.