I'd make a turn-based RPG with battle style similar to X-2, but pre-assigned job classes like IX. The story would be based off of the story I'm currently writing. I would also have "missions" that you can accept for extra money and points, similar to FFTA and FFTA2. I would let the player pick between the 8 protagonists who's story they wanted to follow, but I would let them switch between active members in the party which one appears on screen, and have NPC's react differently to different characters.I would also let players teleport to locations they've been previously. Probably also have combo attacks.

The ability system would probably also be similar to X-2 where you choose which ability to learn and it unlocks new abilities. I would probably also let players transfer abilities from one character to another, and it would have a grade next to it. Like transfer ability A to character 2 will rank a C, which would affect the effectiveness of the ability when used by that character. I would also have an skill system close to Suikoden III where you can use point to boost up each characters stats, which would affect how much they go up during level up.

I think I would have a party of 4 in each battle, and allow them to switch out during the battle. The world would kind of be like XII but with more towns and cities in between instead of just area after area of battles. But it would be free to explore anytime.

I would also implement a character relation system between protagonists and secondary characters. For the 8 playable characters, they will have interactions that will boost their affection points. Things that would also influence it: People in battle parties together, healing each other (or attacking to lower points), choosing them to dispatch on missions together or to bring with you on missions, who you speak to the most during events, etc. It would influence scenes, how effective the magic they use on each other is, and combo attacks. For secondary characters, how you respond to them would affect their affection, which would unlock new missions, or allow them to help you in certain situations, or they could give you presents or things.