HYPOTHETICALLY
What ways could the player be benefit from reaching level 50 and/or complete all achievements in the Factions without giving players an upper hand over one another?
Perhaps;
Order of Souls -
Passing on some basic knowledge of conjuraion and seer magic. Holding a Captain's skull up will guide you to the captain's buried treasure. Would work like the Fate of the Morningstar Tall Tale but with a villanous skull and harder to track.
Blending in with skeles would be fun but OP...
Gold Hoarders -
Not wanting to even take that castaway chest, Jimmy it open like your in Skyrim breaking into a house. Find a random amount of gold and sometimes nought but dust as a nice slap in the face.
Smell Gold in the air, faint whisps of gold slowly seep from the ground indicating trinkets in the area.
Maybe a metal detector like shovel.
Merchant Alliance -
Adept in catching animals, snakes, pigs and chickens are harder to startle and aggrivate meaning easy capturing.
What do you want from me, I wish MA was more fleshed out, I do, but you can blame Rare for that....
Hunter's Call -
Use Pork/ Chicken to lure sharks and other new creatures
