Depends on how the story goes, if the mc becomes king early on, the focus could be on the threat from outside forces, or political intrigues from marius old faction, factions which are against the mc/his politics.
Maybe Marius manages to flee and shakes hand with some other country to reclaim the throne, or Marius in the end, was just a puppet for other powerful player.
Personaly i would like it, if Marius becomes king and there will be a civil war.
Marius being good at pissing people off ? not possible
But yeah, most off the important characters are pissed off by him.
That's including his sister Aelinia, which he pretty much threatened, with what he will do to her when he is in power, or his other sister Cassia, if she finds out that he was behind the kidnapping.
MC & Nicabar the most renowned generals, with the 2 largest armys and MC being the commander of the kingsguard.
Marius has no talent/experience in war, strategies or diplomacy, add to that, being deceitful, his greed for power and hybris.
I think Commodus had a safe work environment, compared to Marius.
But it will be interesting to see, which fate awaits him and who will be the executor.