There are a few things in MLP: FIM that won't happen. I'm not saying that these can't happen. But in my perspective it would be impossible. I'll state three now and some more later.

1.): A duet with Applejack and Rainbow Dash.

Why is this impossible?

It's impossible because Rainbow Dash and Applejack have the same singing voice. Unless Applejack or Rainbow Dash get a different voice atress for their singing roles I can't see it happening. I know there have been group songs where Applejack and Rainbow Dash are seen singing at the same time but those were probably recorded at different times.

2.) Sea ponies appearing in the show.

Why is this impossible?

It's impossible because we've seen the Cutie Mark Crusaders swimming before and there were no Sea Ponies down there. And if there were Sea Ponies by now they'd have an appearance. My theory is that Crystal Ponies replaced Sea Ponies in this generation it's possible.

3.) More Elements of Harmony being discovered.

Why is this impossible?

If there was going to be more than six Elements of Harmony then they'd find out by now. Seeing as this hasn't happened I can't see it happening unless there are a secret set of extra Elements of Harmony. Which I also can't see happening. Although Shining Armor and Princess Cadence both wield the Element of Love this is only on the cards and not official in the show so I don't count that.

If you think some of these things are possible then please do say so.