During Psx-Ps2 eras if you asked me this question I probably would answered something about a turn based RPG mixing elements from Suikoden, FF, Persona...
But right now my dream game would be something open-world like skyrim but with even more role options, like the possibility to be actually evil, not just random acts of violence but ditching aside completly the main quest and conquer the country. And for gameplay...something like dark souls not in difficulty or game design but in how you can build your charácter and the variety of weapons and big charismatic bosses.