First off, i think D4 does not have support gem system ala poE, Torchlight Infinite, undecember in Beta. D4 uses a skill tree system similar to like Grim dawn, & last epoch.
But I think this can be something to add at release, oepn beta or down the pipeline.
For people unfimilar to support gem/rune system, Basically, instead of a skill tree to customise your skills e.g increase explosion area of fireball, split fireball to 2 smaller ones etc.
You do that via gems, like a gem that add extra cold damage, reduce resouce cost, increase projectile numbers, add more numbers, do more damage against elite. They are cool as you can mix and match for many cool combinations & added a layer of progression (e.g gems leveling up)
the limitation is they usually has to be generic (like more damage, more projectile) & usually you cannot completely change how the skill work, that you can in a skill tree system say like some runes in D3.
They also usually end up with mostly buff damage, with a few that affect gameplay, & how skill work.
But what if we combine them. If so, D4 can also be the most in depth skill system.
What I see is skill tree giving buff that altered how the skill work (like converting elements, like changing from cooldown skill to non cooldown etc)
while gem system provide more generical buff like reduce resources cost, reduce cooldown, trigger another skill, increase AoE. increase cast speed, increase crit chance etc.
This will make D4 skil system highly customiseable, & the gems can also add as filler as you level , like reduce reosurce cost, increase crit chance, until you get more from paragon or gears.
