Hi and welcome!

All mods allow you to have up to 5 characters; this is a standard feature of multiplayer. You shouldn’t have to worry about that 😉

As for the rest of your question, I’m afraid most of the mods which truly expand on the storyline are quite RP-centric. I’m not aware of all the mods around, but things such as Discovery or Shattered Worlds War-Torn are RP-focused while expanding on the initial SP storyline.

I’m sure there exists mods that are PvP focused and still expand on the storyline, but for the most part if you want something that really involves the storyline, RP is the way to go.