Maintaining the storyline

If the main objective is to save the world, why on earth (or whatever planet) would you want to rescue BOB.

BOB (or his family and friends) would have to persude us to rescue him.

But were too' busy saving the world, we haven't got time to rescue BOB.

In some way (yet be more stile than my example) you'd have to persude the player to rescue BOB because it would help you save the world.

e.g.
"Rescue BOB from the castle and I'll give you that world cure thing-e that you need."

Apart from this is a RPG. So you can put 'rescue BOB' in as a sub quest.

And thus the player only has to rescue BOB if they feel like it.

But if your game has no actual apparrent objective, you have to make the next step logical.

it wouldn't be:-

    fench a pale of water,
    save the universe?,
    eat lunch,
    go home.

The 'no apparent objective game' leaves you guessing what it is you have to do.
So you wonder the world and keep going and eventually you decide to fetch a pale of water from up the hill. When you get the pale of water it talks to you and says "No, no, the poor universe." And you get confused by this crytic code and eat lunch and go home.
(hey it could happen)

E.G. FF7 had no real one objective, and to be honest was it save the world, stop Sephorith (maybe by letting him help us), stop that evil power company or what?

//honestly, and I consider myself a rpg expert...

The only thing that really tells you is the end baddie. (apart from the ending in FF7 kind of confused you more)

The ending, keep the ending relavant to the game.
//maybe make it so you kill the end baddie and you can still go round talking to people, to find out the ending that way?

Don't change the storyline half way through.

Remember:- It's your RPG so just make it so you like it, and therefore other people like-you will also like it.