I got this idea recently and I thought Yanfly's Plugins could do the trick.
I just played Pokemon Sun & Moon Special Demo and I really liked the new feature of showing what move is effective or not against certain Pokemon with weakness to type matchup (also like Person and the Shin Megami Tensei series as well) where under the attacks in the corner it will say "Not Effective", "Effective", or "Super Effective" on enemy types and if it were possible to do with some tweaks.
Like when you highlighted on an enemy and there's a note saying your skill/spell/power/etc is effective against them or not if you're fighting hordes of enemies and you're highlighting one of them. Especially if you have a lot of types and elements like the Pokemon games with the weakness and helps ease people in case they forgot which is weak to what. Like have the note or notification under the enemy when you select them or display next to their names in the text box above in the default setting... whatever is the most convenient.
I kinda had this idea of my RPG with three types for weakness involving Weapon Types, Element Power, and Melee & Magic-based moves. Basically if you align two of the weakness together the Move Effective note will tell you it would be 2x more damaging if you hit an Enemy that's weak to both Slash-type attack and Fire. Or three if I include it's a Physical attack instead of a Magic attack so they're more prone to Physical damage.
Example would be "Effective 1.5x" for one weakness found, "Effective 2.0x" for two and so on so forth. Perhaps have a background image like an exclamation bubble under the number and it gets bigger or a different color to show the more effective the attack will deal if all weakness are hit along with the number expanding with the bubble or changing colors.
And vice verse if you got the wrong weakness or if the enemy has a second resistance that lowers the effective of the skill. Say for example you use a Sword Skill that is a Slash-type with Ice-element, but your enemy is weak to Slash-type attack but is resistance to Ice-element. So it reduces or cancels the overall effectiveness of the skill's power... to add an extra depth to battles. Same with it being a Magic-focus or Physical-focus skill so if the enemy is resistance to all three than, depending on how you set it up, you can reduce the power of the skill to a tenth of what it is or make it 0 having no effect at all.
This could work with other battle plugins where if an enemy will take the hit for its teammate and cancel your attack as well or reduce the damage. Something simple, but could be complex if one chooses it to be. Ties in with this sort of "Clash Battle System" I had in mind for a couple RPGs I wanted to make with both Allies and Enemies use TP to perform certain actions in the middle of another character's turn/animation/etc and stops or steals or clashes with them if they win a sort of QTE battle or if they have enough TP to perform it. I'll talk about that later.
Just a suggestion that can be tweak to whatever the person wants so it can be customize anyway a person wants. I wanted a make a battle system that works with these three types setup and playing that Pokemon Special Demo on Sun & Moon just inspired me to come over here and support Yanfly with this idea but sadly not capable at the moment with the current Plugins they have.