I can think of a few ways, purity and honour seals, veteran badges all that alter your stats and effectiveness in combat. Moving from Scout to Marine to Squad leader to Apothacary to Chaplain etc as you progress, also allowing movement to Terminator armour at a certain level. Also leveling to new weapons as you gain skills, ie bolter and flamer as base weapons then plasma and missile launcher, then heavy weapons and better close combat weapons like power fists and chain swords/force weapons. You also have the grey nights and psykers to consider as well. I can see plenty of scope available to level up mate.
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