Alright I'm gonna go over a loyalty system that should work for marines. Realistically, each rank should have different prerequisites for ranking up, and should positively add to the gameplay experience for the marine faction, being fun and rewarding, but also progressively more challenging as you rise in rank.
Starting out as a marine, your first goal should be to locate either a marine camp, a marine base (large or small), or a marine warship. There, chore boys will right click on the marine captain in charge, and will be given a list of simple tasks such as : Retrieve 100 logs, 100 stone and 3 blocks of coal, and 20 of any raw meat. Upon completing these tasks, talk to any marine captain, and you will be promoted to Seaman First Class, and be rewarded with 500 belly and a marine uniform (hat top, pants, boots)
Following that, Talk to any marine captain, and he will assign you to kill weak mobs. a task such as : Kill 10 skeletons, 10 Zombies, 5 pirate grunts. Completing that task will cause a "marine letter" to appear in your inventory (first available empty slot in your hotbar, or if thats full, first available empty slot in your inventory. otherwise, it is dropped on the ground. in the event a play forgets to retrieve it, the letter will spawn again at the start of the next day cycle, until the player picks it up and uses it) Right clicking the letter will promote you to Petty Officer.
Upon promoting to Petty Officer, you will receive a letter called "first mission" in your inventory. (same conditions as the marine letter applies) in it, you will receive a gathering task such as: collect 30 bullets, 30 gunpowder, 1 marine sword and 1 senriku. Upon completing that task, wait until the beginning of the next day cycle, and you will receive a letter called "second mission" (same conditions apply as with other letters) this will be a "rescue" mission. The task is to work alongside another marine npc to defeat a pirate brute, with no casualties. (the mission requirements trigger the you enter combat with a pirate brute. Any marine npcs that join in, or, that you join, will become a part of the mission)
Upon completing this, on the next beginning of the next day cycle, you will receive a letter called "third mission"(again, same rules apply as with the other letters) this mission will be a pirate specific combat mission. the task will be to defeat 5 pirate grunts and 2 pirate brutes. Upon completing this task, you will receive another marine letter. right clicking it will promote you to warrant officer, and reward you with 1500 belly, 10 sea king meat and a bazooka. As a warrant officer, you have to visit a large marine base, and receive a task from all 3 of the marine captains stationed there.
The tasks will be: kill all enemies on a small pirate ship from the first captain you speak to, retrieve a buried treasure map from the second captain you speak to, and the third task from the third captain you speak to, is to find 1 diamond.
upon completing all 3 tasks, you will be promoted to Ensign, and be given a marine jacket (basically like smokers jacket, but more basic, and has the sign for "justice" on the back.)
and 2000 belly, as well as the skill "small muster".
At the beginning of the next day cycle, you will receive a promotion mission letter(same rules apply) right clicking it inside of a village will trigger a small raid consisting of only pirate npcs. (raid hp bar is half of a normal pillager raid, and only has 1 wave.) Upon completing this task, on the beginning of the next day cycle, you will receive a promotion letter(same rules apply) right clicking it rewards you with 2500 belly, 2 golden apples and a Jitte and promotes you to Commander.
At the beginning of the next day cycle, you will receive a special mission letter. the task is to comfirm the existence of potentially dangerous areas that require further investigation from a marine admiral. Your task is to visit the nether, an ocean monument, and a stronghold.
completing this task immediately rewards you with a marine captain coat, a wooden box, 1 key, 3000 belly and a promotion to Captain. at the beginning of the next day cycle, a "request from HQ" letter will appear (same rules apply) right clicking it, gives you the task of killing all enemies on a medium pirate ship with a small muster of marines, and destroy the ship (destroying 20 or more blocks on the ship will satisfy the mission requirement.) upon completing the task, you are immediately promoted to Commodore rank, and are rewarded with 3 enchanted golden apples, 20 lapis lazuli, and 20 minecraft levels worth of exp.
At the beginning of the next day cycle, a request from HQ letter will appear in your inventory. Your tasks will be to uncover the bandits secret storage cache and defeat all enemies inside (find an underground bandit base). Upon completing this task, you are promoted to Rear Admiral and are rewarded with 4000 belly, A protection IV book, 10 sea king meat, and +500 doriki.
A letter will not appear this time. Instead, the prerequisite for promotion to Vice Admiral, is to possess Kenbunshoku Haki and Busoshoku Haki. When you unlock both of those, you are immediately promoted to Vice-Admiral, and are rewarded with 5000 belly, 20 sea king meat, Vice-admiral clothing(top, legs and boots)and unlocking the "Muster" skill (using the marine captain coat in crafting with a vice admiral suit, results in a vice-admiral complete suit. the complete suit has 10 armor and 5 armor toughness.) At the beginning of the next day cycle, You will receive a secret mission from HQ letter in your inventory (same rules apply) the task will be to retrieve and open a golden box from a large pirate ship, with a muster of marines.
Completing this task will promote you to an Admiral, and rewarding you with 10 diamonds, 5 enchanted golden apples, 1 golden box, 1 key and +500 doriki. After 7 day-night cycles, at the beginning of day 8, you will receive "the gorosei's orders" which will task you with defeating the wither boss, the elder guardian, and the ender dragon. Completing those tasks will reward you with an immediate promotion to Fleet Admiral, 10,000 belly, a golden den den mushi, and a hito hito no mi, model: Daibutsu.
Very clear lines of progression, and failing a mission will demote you to the previous rank. Very simple, and very fun instead of just playing the waiting game