I took a course on Udemy to learn about the Gameplay Ability System, created inside the Unreal Engine, for the game Paragon.
https://www.udemy.com/course/unreal-engine-5-gas-top-down-rpg/learn/lecture/37324564#overview
Its was certainly the most extensive and advanced course that I have followed on game development.
Rather than simply finishing it, I took another 3 months to make my own RPG Systems. The most complex one being a skills talent tree system inspired by Last Epoch.
And I am pretty proud of the result.
Each skill has its own talent tree allowing you to influence the skill in a unique way. Such as adding more projectiles, making special effects trigger under certains conditions, influencing the damage upon custom conditions (as health above or below a certain threesold).
If you wish to try it for yourself, it is available here : https://drive.google.com/file/d/19FYRvVGp1wAkqexLSU2flGne9b9yfkss/view?usp=drive_link
And if you are interested in the code :