The standard model is not a complete model of all elementary particles. It is a model of all known elementary particles.

There are reasonable outstanding predictions for:

- More Higgs bosons

- A Graviton

- Another 17 super symmetrical pairs to the existing 17 known particles.

- One or more dark matter particles

We don't know which (if any) of those will end up coming true, but they could all add new particles to the standard model. It is also entirely reasonable that new particles outside of the ones I listed could be added and incorporated into the model.