No - there really can’t be.
An element is determined by the number of protons in the nucleus. We’ve found (or artificially synthesised) materials where the atoms contain 1, 2, 3, 4, 5, 6…all the way up to about 116 protons without a gap. So there aren’t any out there with less than 116 protons that we don’t already know about.
So - what about the elements with more protons than that?
Well, the more protons you stuff into the nucleus, the faster it falls apart. Elements up over 100 or so are horribly radioactive - and most of them break down into smaller atoms in a very, very small fraction of a second.
There is an idea that atoms that are even larger might get more stable and survive for a while longer - but making those atoms seems impossible with any feasible tech.
So no - there aren’t any elements (that are remotely stable) that we haven’t already seen.
The new elements synthesized are prepared in nanogram quantities or even much lesser than that due to the poor stability of artificially prepared nucleus. So basically discovering an element now means synthesizing it.