Can't wait to find out what the differences between the Melingoi and the Ezeritai options might be, if any.
Joking aside, I think I know where I'll start in my next Byzantine game now.
There are actually some slight differences between the Melingoi and the Ezeritai courtiers, though nothing too huge so it's more a flavor thing. The Melingoi get a bonus +2 martial while Ezeritai is +1 martial and +1 intrigue (both have +15 combat rating) (bonuses subject to change):

The more important difference is that when the troops are levied via decision, the Melingoi will have slightly more heavy infantry and cavalry, and the Ezeritai more light infantry, archers, and pikemen. (Though you can levy both if you pay more)
None of this is exactly historically based other than the fact that the Melingoi apparently paid more in tribute to the Byzantines historically, so based on basically no other evidence I've just assumed for the sake of the mod that they're more prosperous and the Ezeritai are poorer and more rugged.
There will probably also be a similar sort of differentiation for the Tsakonians and the Maniots.