Still, if you could make a game with that kind of budget, how would you develop it?

I'd make the biggest most fleshed out and content packed (J)RPG ever.

I am not a game designer and do not pretend to be. I am a consumer of the hobby.  While I work for one of the largest software companies on earth, I am not qualified to answer your question.

I have bad news for you...

I'll produce Final Fantasy VII remake, and the sequel covering Genesis and Weiss.

I'd spend 100m on Legend of Dragoon 2. Then 400m on hookers and blow. You know, the exact same thing Bobby Kotick did with Destiny.

I would just give the money to R* so they can make GTA VI any way they want... 

Assuming I do not have to care about market realities, I would make a JRPG with massive scale and cutting edge visuals. Battle system would be Grandia II (because Square Enix sure isn't doing anything with it!) based and could have couch co-op where character control be split how ever P1 and P2 wanted. Be able to have tons of side quests and new game + and hidden super dungeons.

Story would be darker with more nuanced characters and seeing various factions and threads woven together as you play across different parties. The musical score would have to be full orchestra with plenty of epic movements in the score. Character design would be bold and distinctive with some classic JRPG clothing choices.

So, I would be so happy and my company would probably go brankrupt.

I know cutting corners and delivering bland content is not how Rockstar works. But you trust them that money and they don't deliver the game you expected. What now?

EPIC.