It would work alot like an RPG like Witcher or Dishonored
Starting off-
Welcome to Mistborn Birthright *Space to begin* Start new game?
*Cutscene*
Select your race:
Kollos-blood---Terrisman---Kandra---Human
*Customise character*
Depending on who you picked
Koloss-blood. *passive* 50% strength boost, 20%stamina boost, 40% Health boost, +10 dmg (increase every level) when using melee weapons. Can be Mistings only. ABILITY- Rage. +10% boost to all stats (Duration 4 minutes) Can be levelled up. SP on start 3.
Terrisman. *passive* 20% intelligence boost, Can be Full Feruchemists or Ferrings or Twinborn. ABILITY- Pacifier. Can pacify enemies and stop them from attacking you. SP on start 6.
Kandra- *passive* +30% Stealth, +20%Speed, ABILITY- Shift. Can take on the appearance of whoever you killed last. SP on start 5.
Human- *passive* 20% intelligence boost, Can be Mistings, Mistborn or Twinborn ABILITY- Concentration *passive+active*. When in concentration, time slows (to you), and you can use your environment to your advantage.
Each has their own skill-trees.
In game you would use WASD to move, LMB to attack. RMB to interact/ zoom in. Scroll wheel to switch weapons. E would change the metal you are burning/tapping. and Q would activate each metal's unique abilty like @Eki said. ZXC can be used to activate the other metal's abilities, which you can equip by holding F, which will stop time and allow you to pick your metals. R would reload your ranged weapons etc etc