In my opinion, new raid bosses should include:

Level 1: The middle forms of the Hoenn starters, as well as Feebas.
Level 2: Arbok, Raichu, Clefable, Wigglytuff, Jynx and Porygon
Level 3: Golem, Ampharos, Espeon and Umbreon
Level 4: Aggron, Flygon, Slaking, Salamence and Metagross
Level 5: All Kanto, Johto and Hoenn legendaries and mythicals not currently available.