[SkittlesAreFalling]

Start:
What are the requirements for the mission?
- Skill.
- Materials. [Vehicles, Weapons, etc.]
- Score.
- Completion of another mission.

How would they start their mission?
- Checkpoint.
- Command.
- Goal.
- NPC interaction.

The Mission:
What are they doing on their mission?
- Killing.
- Stealing.
- Breaking in.
- Delivering.
- Rescuing.
- Racing.
- Much more.

What is the setting?
- Object placement.
- NPC placement.
- Checkpoints.
- Animations.
- etc.

Ending:
What is their reward?
- Experience.
- Money.
- A perk.
- Score.

Tip: When a player is on a mission, I suggest putting them in a different world, so there's no interruptions from other players (who are not on the same mission).

This is the pattern, now to make it dynamic, use selectable textdraws or dialogs so developers can go into the game and set-up the mission from within the game.
Since you are limited to the amount of data being stored in the script, I suggest using SQLite to help make an endless amount of data.
I've made a system like this and will do it again if you need help.