You start as a powerful ruler of land within the republic, such as the owner of a barony. You then get to elect yourself as the Doge, trying to bribe your way up with all the votes. You will be competing with prominent merchants (trade system that comes with the Republic DLC) and other land owners. If you ever lose the Doge, you just go back to being the Baron of your land.
Now, instead of forcing the player to change dynasties (which goes against the game concept), you just have to allow Barons to be playable (much easier and flows with the game).
That is just the basic concept, which can be built upon however paradox wants to. This concept might even be moddable already, or at least easily moddable with the allow_baron = yes command is implemented.
A republic DLC is not complete without a trade system. This can be anything from the EU4 system to a completely new system. I'm just going to make one up here to show how simple it would be.
Each Merchant Republic gets 3 trade routes. Depending on the highest title you have, how much land you have and important trade cities which you control, you get more. Each trade route has a "Merchant Fleet" attached to it, which will work like a Barony, only over water.
You can attach these fleets to provinces that you want influence over. Once you reach a certain amount of influence, you can do special actions, such as invading and conquering the cities, kicking other merchants out of there, or even launching a raid.
Now, here is where a player in the Merchant Republic comes in. Lets say he owns 1 barony and 1 merchant fleet. While he is plotting to become Doge, he can expand the influence of his Republic into Greece if he so chooses. He can decide to go on a Pirating mission against the heathens in Africa if the Republic is low enough in authority. Or he can just sit back and play the marriage game to gain more influence around the world.
The whole point is, a Merchant Republic DLC will require:
- New Inheritence System
- Trade System
- Overhauled Navy Mechanics
But that doesn't mean that it couldn't be done in segments. Just the bare bones can be added in right away. The rest has been asked for time and time again, and should be added anyway, regardless of a Merchant Republic DLC.