MII TW introduced the possibility of static and dynamic events - in my opinion one of the most important improvements over RTW.
Over the last months I have thought about the possible use of these, perhaps this is useful. :)

Below are some of my ideas.

(1) Economy

At the start certain factions have control over only one province, leading to the problem (in EBI) that the player has only two realistic options - blitz until the income of the growing empire exceeds the upkeep of his forces or disband the starting army. In EBI there is - due to engine limitations - no way of turtling with such a faction AND keeping an army that is able to defend against intruders. Examples for this situation you can find with the celtic factions (except the Casse).

What about this system instead:
At the start such a faction gets a significantly higher King's purse.
When the faction grows and the player gains control over the (for example) fifth province a static event is triggered which tells the player that now - with much more people to control - the administration effort has significantly increased and therefore a greater amount of the income has to be spent for this.
Consequence: the King's Purse is (again: for example) halved.

This system could be pursued even further and could help to prevent the general RTW situation that your faction always gets filthy rich after a certain point in a campaign.
As far as I know the king's purse can be set to a negative value even. Why not use this to substact some of the money you otherwise would just find in your ever-growing treasury?
Ideally this could help to make decisions about how to spend your money difficult even in later stages of a campaign.

(2) New (and reformed) units

There is a description for one of the SPQR fleets (Quinqueremes iirc) that says that the knowledge how to build these ships has been stolen from the Phoenicians (one of their ships was thrown to a roman shore during the First Punic War).
What about making this an event - as soon as the SPQR wins a sea battle against the Phoenicians (afaik the actual type of the ship can't be used as condition, at least I didn't find anything when I searched) the player gets a message saying basically what that description in EBI said and he can build those ships.
Perhaps even better - make it a "yes/no" event and ask if the player wants to spent a good sum of money to pay roman shipwrights to research how this ship can be built.
And if the player selects "no" (for example because he lacks the money at the given time) he has to defeat another phoenician fleet to get this opportunity again.

The reformed units work similar, it's just the trigger that is different. But I'm perfectly sure that you have plans to use events for your reforms ...

(3) New building options

When I played the Sweboz I was a bit disappointed that they aren't able to build large walls (the ones you can put troops on).
Of course this is historical accurate but on the other hand - oppida walls were within reach for the sweboz, as far as my knowledge goes.
I don't mean they were able to build it and just didn't do it, I mean they were close enough to the celts to see the advantages.
What about an event that triggers by the conquest of Vienna, Vindelicoppidos or Gergovia? (The names are just "placeholders" ;-) ).

Similar to the ship event above the player could be offered the opportunity to let research (and pay for it of course) the way these walls are built and in the end gain the opportunity to erect these walls himself.

(4) Special units with restricted availability

There are several units in EB that historically were rare at best (the goidilic shock infantry which was even removed due to this, the germanic wild men - Worgozez? etc).
Why not restrict them to 1 unit per campaign?
I think it could be possible to make them appear only by an event - example for the Worgozez:
The event pop-up describes an atmospheric campfire scene somewhere in the northern woods (the player has to send a family member into these woods to trigger the event) where that family member stumbles upon a band of wild warriours who are - for the right price - willing to be hired into the faction's service.
If you make this a one-off-event you have a unit that is available exactly one time in the whole campaign.
You could make it retrainable in the settlement of that province (and even combine this with an expensive building that is buildable only after that event and which is necessary to retrain the unit) if you wish.


(5) Simulation of the split between the Arche Seleukeia and for example Baktria

Reconquista TW (the german mod) had a system (a simple "yes/no" event) that simulated the emancipation of Burgundy from the HRE. The (Burgund) player basically had to decide if he wants to become/stay in a vassalage to the HRE or to break off. Vassalage meant a decrease in authority and King's purse as well as becoming allied (or even a protectorate? ), to break off brought an increase in authority and money but meant war between the two factions and spawned a stack of HRE forces near Burgund's capital.

This system could perhaps be used for AS on one hand and the satrap factions (like Baktria) on the other?

(6) Invasions
Self-explanatory. Just inform the player that some tribe/faction is rumoured to storm towards the mediterranean (or whereever their destination may be) and spawn some stacks. Much like the mongol invasion in the vanilla MTW or the Yuezhi invasion in EB I.

That much from me for the moment, as I said - I hope this is rather helpful than annoying.

[Hm. After re-reading I get the impression that these ideas are obvious to the point that posting is pointless but I will share them nevertheless. ;) ]