Ah. I would summarize those under game mechanics. With system I think stuff like seen in turn based combat. The real time combat is the system to me: real time action. But this is just me, perhaps.

Well obviously you need some type of resources and ways to manage them and improve them by skills/equpment and so on. Often seen is the red blue green type and everything just modifies how much you gain or lose of those. Health, Mana, Stamina. Can get more simple or more complex. Like adding armor to health. Or adding sanity, gold, hunger, charges, ammunition, special-attack-bar and whatnot. Or removing Mana/Stamina or both.

The more complex your system gets the more complex will the balancing be. Is there one winning strategy that is better than everything else or are there more strategies that will work. Are there classes that have bonuses or even exclusive mechanics.

The list goes on and on. There is no simple answer to your question. Or rather, there is a plethora of answers, since there is a plethora of first person action combat games that already demonstrate such systems.