Well, it's a big solar system, so there could definitely be some large planetoids hiding out there. And almost certainly 1,000s (100s of 1,000s?) of smaller objects in the Oort cloud.

So, what effect would they have on us? None. Nada. Zip. They're simply too far away, by several orders of magnitude. Will they get closer? Nope. Any large-ish planet that orbits close enough to the sun to be destructive (or even visible, really) would have been detected long ago either through direct observation or (more likely) by how it affected the other planets we do know about.

No unexplained glitches in the orbits of the inner planets = no unexplained planets that have ever visited the inner solar system.

Now, the far outer solar system is a different question (70+AU). There could be (and probably are) all kinds of exotic and weird objects out there just waiting to be discovered. I suspect that Sedna is just the first one of hundreds we'll eventually hear about as observation capabilities increase. And that doesn't include the 1,000s upon untold 1,000s that are simply too small or dim to see at that distance.