Rebels
We all know how a rebellion works in Rome 2. Instead, it should be better if:
A random stack from 14-20 units are spawned at once (1 turn). There should be a balance stack with only few cavalry and the units should have random experience too (from 0 exp to 4 exp). Of course, there should be differences in the rebels of every faction (for instance you expect to see more infantry in a roman rebellion). Public order should go from -100 to 0 (1 turn). After crushing a rebellion there should be +10 for the next turn.

Diplomacy
As you go larger and larger everybody hates you. So, if you have client states or allies, you might have to kill them, as they tend to turn their back to you. I am not say that expansion should be easy, but it should be better instead of having minus relation with someone, only because have a huge empire does not make any sense.
There should be a huge penalties when you declare war to someone, and after the end of the war should take many many turns to have good relations again. However, if you turn someone to a client state or satrapy the penalties should be less, since you allow them to remain in the map and have autonomy.
Make harder to take defensive or military allies (for the humans only)

Taxes
Taxes in my opinion should work as follow:
Very Low taxes: +7 public order, +5 growth -50% tax, Cultural/Religion conversion +2
Lower taxes: +4 public order, +2 growth -25% tax
Normal taxes: -4 public order
High taxes: -15 public order, -5 growth +50% tax, Cultural/Religion conversion -1
Very High taxes: -20 public order, -10 growth +100% tax, Cultural/Religion conversion -2

Basically, player will not raise the taxes many times during the campaign, as we do in the vanilla.