Personally for me anyway.
In Short: Yggdrassil from Overlord LN Series but less predatory, or just a better Black Desert Online really.
In Long: A Beautiful mix between the expansive system of Turn-Based RPG and the Action from ARPG, with an actual Role-Playing System from Simulation games like the Sims etc.
Like let say the expansive system like Turn-Based i mean imagine DnD Skill/Spell System really, like on top of a big roster of skills and spells, many different classes, unique build, also don't forget Spell Modifier or Skill Modifier, with strength and weakness system like from idk Shin Megami Tensei, or Persona and so on.
Action from ARPG well.
When i was a kid i'm playing this game in Phone called Mystic Guardian, this game you see has a neat system to make up the fact that it was a game designed for phone.
Rather than goes the simple way like Zenonia or Inotia, the fighting in it uses some sort of
Dynamic Combo System so to speak, let say that you attack once, you can choose to attack again,
OR do a follow-up attack, the follow-up attack is different for different state of your normal attack.
I've been thinking about this because i also play this small MMO Games called Toram, this System coupled with Toram Skill Combo System, mixed with the amount of skills you can get from games like DnD or any Turn-Based RPG is one hell of a Gold Mine waiting to be mined.
It's expensive as hell to develop and balanced but hey the purpose of the Thread is our definition of Perfect Game right lol.
As for the Simulation part, it honestly pretty self-explanatory, i don't think i need to explain that one.
Mystic Guardian Gameplay for references:
And Toram Combo System for references as well (Just example for this one):
If it's not clear enough in Toram you basically can create a Combo System where you can Combo one skill after another, so rather than being a Interface-fest it was like in World of Warcraft for example, you can just create a Combo where you cast 9 Separate Skill and with that combo active you only need to cast the "opener" skill of the very first skill in the combo and it does the rest of 8 skill automatically.