>>28214572I don't expect to see fossil pokemon, honestly.
Alola is based on Hawaii, which is a volcanic island chain. It hasn't existed long enough for normal fossils to have formed. Therefore, it wouldn't make much sense for them to appear in this generation.
HOWEVER, some fossil-like items exist, such as the "fossilized" forest of Lava Tree State Monument and several man-made structures buried in the more recent lava flows.