Nidoran Male is an event pokemon in the pier, just need a gourmet treat. Staraptor if unlucky with the mystery egg can be obtained right after
If you are not willing to wait this long, talonflame is an amazing pokemon even post nerf, and so is xatu, which alongside dusk lycanroc, can be obtained before the first keta fight. Crobat is also an option but with the lowered level caps its way harder to get. If you are not that adamant about decidueye and/or repeating types, there is the event budew which comes with extrasensory, alolan marowak can be obtained right before narcissa, vileplume can be obtained as soon as gloom evolves since there is a leaf stone in gearen park and both duskull and yamask are available in narcissa's gym. As one of the people above me said, just use stuff you havent used before, im doing that on a new file and really enjoying it so far