For reasons on why RPG mechanics in action games tend to be a bad mix, read here:
Now I want to showcase a recent game that - completely unexpectatly - did RPG mechanics right in a non-RPG title. Super Smash Brothers Ultimate.
Yes, I didn't see it coming either. Now to note, the RPG system is only really relevant in the game's story mode "World of Light".
You can enable them in multiplayer as well, but it is an option and rarely used or seen.
How does it work?
If you want text, there's a write-up by me below. Otherwise here's an official video that gives a bit of a run-down:
There are three elements, your character, your Primary Spirit and your Support Spirits.
Your character is simply that, the fighter you choose to use.
The Primary Spirit is a character that gives you 'power' so to speak. It has a defense and offense stat that have a minor influence on the game. If your defence stat is lower than that of the enemy's offence stat, you take bonus damage and vice versa. If your total stat (defence + offence) is lower than that of the foe their AI is increased proportionally. So stats mostly play into how hard the CPU is. The Primary also can give a minor buff, and if he's from the same series as your fighter you get a minor stat boost. For instance having the Bison Primary while using Ryu, gives you a nice 10% bonus.
Then there's the Support Spirits. The Primary Spirit dictates how many Support Slots you have, maximum being three. Some Primaries only have 1 slot but huge stats, or three slots but a negative modifier (taking bonus fire damage for instance). The supports can vary wildly, some take up one slot and give you bonus damage on for instance a Neutral Special attack. Others give bonus PSI damage (Lucas and Ness only), or increase Jump height or let you start with a certain item. Others take up two slots but give you a big bonus like an auto heal if you take too much damage or a third jump, while some even take up three slots.
The beauty of the system is that fights in the story mode have weird things going on, which spirits can solve. There's a fight where the floor is on fire, so you can equip a "decrease fire damage" support, or a stage where the enemy is of Metal constantly, where equipping the "Metal Slayer" support helps out. You can build a lot of mechanics around this, focusing for instance on a Captain Falcon that has a ton of buffs to his Falcon Punch - but can't offer a slot to negate stage-hazards. You can also negate your setup to negeta stage-hazards if you wish.
There's a ton of fun optiosn to toy around with, without going too overboard and making it so that stats win you the match. Instead spirits can be leveled to increase their potency (stats) to lower enemy AI, or even evolved to new forms with different slots or bonusses. It is a really fun system and I'd love to see something like this in an action-game, I think it would work so insanely well.
Here's an example of a spirit screen:
This setup is pretty great, he's got 4 times "+fist attack" which could work fantastically for Captain Falcon or for instance Little Mac. But it also leaves you vulnerable since you don't have other more utility buffs or ones that negate stage-hazards. Meanwhile the Primary also has a very low defence stat, meaning you'll take a lot of damage. Its total stat noted above is 10.000, late-game fights have enemies with around 13.000 for instance so he is at an disadvantage, meaning smarter enemies.
Really fun system! Thoughts :)?