What might it look like to have initiative modified by combat stance, and by the type of action you last took?
There are a few ideas here, not all of which need to be combined into one system. Any combination could be interesting.
NOTE: in this system, you get one action, free movement like in 5e dnd, and 2 quick actions, per round. Quick actions can be used on turn or off, depending on what you're doing with them. You can also do incidental stuff without using an action, like opening doors and flipping switches or switching weapons.
- I don't like declaring actions at the top of the round, but if you cast a complex spell last turn, maybe that means you drop down the init in this round?
- If you take an aggressive stance, you get an init boost, but either are losing out on a defense boost, or outright take a penalty?
- If you take a defensive stance, you get to do something at the top of the round, but take your full turn at the end of the round? Or maybe get an extra quick action or something?
- Maybe a stance where you get no bonuses except that you can insert yourself into the init order at the end of another creature's turn?
- A protective stance would help you "tank" by imposing penalty to enemy action, but go toward the end like defensive stance?
- Perhaps certain combat skills give a bonus or penalty to initiative if they are what you have ready to fight with?