Keep seeing this Everon Cartel tag, thought I'd find out what it is. Fantastic!
Few thoughts and ideas.
If this campaign is going to be based around mone character (like OFP:R) then its probably a good idea to make him a strong, believable character - the player needs to believe that he is that that character. Give him a home and some friends in the village and make him go back to meet them regularly. Maybe have little mini-missions where all you have to do is meet up with your mates, collect some beers and sit on a hillside watching the sun go down - almost a glorified cutscene in fact.
If he has a girlfriend then he meets up with her 3 or 4 times during the campaign (or in in-between cutscenes) and she has an important role to play near the end. In other words do it better than OFP:R when you saw this girl once for two seconds and 19 missions later you're supposed to be upset because she may be about to die (don't want to give away too much plot here ....)
I would go one step further than Rubble Maker and say before you even have a mission tree you should have a plot and character tree. This thing should feel like a small but exciting interactive novel before you even start the mission design. Who are your characters? What happens to them? What you don't want is lots of minor characters who appear for one mission and that's it: a good rule of thumb might bethat every character who has a speaking part must appear at least twice.
If the debate about how you get into the Cartel is still open I propose this. You are a criminal who moves to the island to avoid the heat from the past. Not knowing about the Cartel you decide to do a little smuggling to earn a crust. A small scale turf war erupts on your patch as you unknowingly intrude on the Cartel's operations. The hit squad sent to take you out doesn't come back, because you're good. (That's a mission in itself.) The leader of the Cartel, being smart, decides to negotiate and you, also being smart, quickly join the team. Obviously you have to join at a junior level because you are new, but equally obviously you rise quickly because you are already experienced.
Mission idea. You are hanging out at Cartel HQ when there is an unexpected police raid. You are doing your bit in the fighting when somebody tells you to go and see the boss. You find him and he orders you to use the diversion of the firefight to eliminate one of his lieutenants. The boss says that he has an agent inside the police who has told him that the lieutenant is a stool pigeon. He can't be eliminated in the normal way because that would reveal the source in the police. Naturally this lieutenant has, in the previous missions, been built up to be your best friend in the Cartel. Four possible outcomes of course, either you shoot him or you don't, and he is really a stool pigeon or not. (Perhaps the boss saw him as a rival and wanted him out of the way, the whole stool pigeon thing being a tall tale.) Another possibility, its not a real order its just a test.
Big question which I haven't seen discussed (may have missed it) What is the ending going to be? Do you get killed? Take over the Cartel? Take over both Cartels? Does your girlfriend get killed and you retire, griefstricken, to raising tomatoes? Do you see the error of your ways and destroy the Cartel? You want the ending to be something less predictable than just "conquer the island".
This is a wonderful project, the trick will be not letting get out of hand. Don't go overboard on mods - there will be enough creation and downloading as it is. Probably best if you make sure that all the mods can be used in at least two missions. And non-linearity is great fun but its a lot of work .... If you want an 8 mission campaign with every second mission having two possible outcomes (i.e. in the whole campaign the player gets four binary choices) that adds up to something like 30 missions unless some of the paths join up again.
Anyway, that's enough for one post.
l8er