[quote=@Magnato]
But seriously, I'm just a bit confused, say I want my character to be some sort of swamp monster, controlling slime/water/life, how would I change the elemental table for this?
[/quote]

You could just have Water and Life as your starting elemental cores. That way, you'll have high masteries in Water and Life, which will allow you to use Slime-based (i.e. Water and Life-based) abilities with ease.