I have a few general ones:

No more than 4 foot missle units (archers or slingers) per stack.

Huge/VH-H/General cam on, no pausing (unless its urgent eg phone rings or need to pee).

Disband non-professional or non-elite units between missions.

No mercenaries unless I have a city with that unit's culture. eg no gallic mercs for Pontos until they have Ankara, no greeks for Lussotannan until they have a hellenic city (with a type IV gov).

Armies based around leaders. "Royal armies" for FL and heir with elites (and maybe some elite allies). provincial armies for lesser generals. allied armies for allied generals eg in my Lusso campaign, the type IV gov in Massilia is nominally Greek, so he's a greek merc general with only hellenic troops.

Pillage in a culturally appropriate manner: eg celts will burn the temple of Jupiter in Rome but respect Stonehenge, Romans will drain the lakes in Gaul but respect the Parthenon.

For my "less civilised" campaigns (ie barbaroi) I add a few:

No more than 4 units of one type per stack (variegated hordes): this forces me to manage a mix of unit types and diversify to build bigger stacks.

no long sieges: either you seige assault after one turn or you walk away ("the tribes got bored").

can't expel civilised populations (not compatible with the babrabarian home population).

I try to RP a growing state with babaroi (otherwise the faction would collapse with every FL death) so eventually certain elites become regular army and don't have to disband: for Lussotannan its Scorto's and Balearic slingers. I have tied with the idea of tying state run granaries with the ability to support a standing army, but its not a hard and fast rule, just a flavour I've added.

I tried to RP a "revolt against Rome" where I sent a general into Latium, and allowed him to recruit italic units once I controlled 2 italian cities but it turned out a bit of a fizzer as the Rome garrison sortied to support a single unit attck on my besiegers, and got routed.....so Rome fell without even a decent street fight.