I think they are more than likely some fossils out there that we have not yet discovered, that could lead us to find a new species of dinosaurs that we had not previously discovered. As a matter of fact, it would not surprise me if, in the future, we dug up multiple fossils from dinosaurs we had never thought existed.
Our world still has many things yet to be discovered -- more dinosaur fossils being a part of that. We don't actually know very much about our world and we're still finding living species, so why wouldn't we find instinct ones? I think it is definitely possible for us to find more dinosaur remains.
We cannot assume that we have discovered all species of dinosaur. Almost every year in the past fifty years, several new species of dinosaur have been discovered. With improvements in technology, especially genetics, it is certain that more species will be found. In fact, by genetically sequencing the specimens that we have already found, it is highly likely that even more sub species will be discovered.
I would certainly not rule out the possibility that there were many more dinosaur species that we are not presently aware of. Look at the plethora of new things we stumble upon daily when we are not even searching for them. I believe it would not take very long at all for a team devoted to searching out an, as of yet, undiscovered dinosaur to succeed.