A bit of a fusion of JRPG and Sandbox gameplay.

A game in which you can have ANY NPC humanoid in the RPG become a member of your party.

For example, you can go to the university of a random city in the game, you can in a female dorm of said university, you can find the random creepy girl that has painted her entire room black and keeps satanic items around her room and you can ask her to join your party.

Another example, you can be walking along a shore, the high tide comes in, you can't escape the surf and you're carried into the sea, a few hours later you wake up to a merman with long, blonde and wavy hair watching you.  You later ask him to join your party.

Final example for today, a random Zombie is limbing around in a street alley looking for food.  He uses his super human strength to rip open the flesh of rats and alley cats for comsumption.  You later ask him to join your party.

Boom! You're playing a JRPG with a Black Mage-like Satanist, A White Mage-like Merman and a Warrior-like Zombie!  Sweetness!

-Waits 250 years for this to actually happen-

That's an awesome idea, but it would be too much if ANY NPC could join your party. Where would their plot development be? What about their abilities? Why buy anything if you could just ask the shopkeeper to join your party?

I much prefer the idea of every person being an enemy.

The storyline would adopt.  The shopkeeper's next of kin would keep the shop afloat.  The zombie would be the angsty, emotional one.  The Satanist would be the annoying love interest.  The Merman would be obligatory badass that later leaves the party and betrays it.

Sounds like Chrono Cross where you could have a heavy metal guitarist, a luchador, and a voodoo doll as your party members.

hmm i swear i have played some game where i can have anyone join. can't think of it though. its probably some game online that's free.

but one thing i'm annoyed with in rpg's and party member games are HOW DAMN OBVIOUS the joining guys are. You can always peg them basically the moment you see them as new members. Try and surprise a bit here dev's

That was done once. The game is called Radiata Stories

God damnit.

God Damnit..

I was going to mention that, but I would also have had to mention Chrono Cross too, and that game's characters should not be mentioned.

Chrono Cross is a bit different, it's like Suikoden, they just have large casts of recruitable characters, way larger than any RPG out there (40ish in CC, 108 in Suikoden). In Radiata, every character you could talk to was either recruitable or antagonist

I'm talking everyone.  This idea would be similar to Radiata Stories but every character is recruitable.