As we all know, the Raid Battle system is currently in a beta version. Only Trainers Level 25+ can participate in Raids, which are now no longer at only sponsored Gyms. But now that Raids are available for most Trainers, what is really being withheld during the beta? Beta versions are normally versions that do not contain every feature, so it makes sense that Niantic has not activated every Raid Boss yet. In the promotional screenshots the following Pokémon were shown as Raid Bosses: Dragonite, Ninetales, Charmeleon, Gyarados, and Blissey. These Pokémon, along with Legendaries, have not yet been spotted. Many other Pokémon (list below) make perfect sense to be Raid Bosses, as well.

Expectations  for  possible  new  raid  bosses

golem

nidoqueen

espeon

umbreon

aerodactyl

ampharos

feraligatr

pidgeot

clefable

nidoking

steelix

omastar

typhlosion

donphan

granbull

kingdra

wartortle

crobat

ninetales

meganium

kabutops

gyarados

charmeleon

ivysaur

blissey

houndoom

wigglytuff

ursaring

togetic

dragonite

Please keep in mind that the list is PNN's speculation and not yet confirmed. It is also worth mentioning that Scizor, Espeon, Ursaring, Steelix, Kingdra, Umbreon, Granbull, and Houndoom are the least likely future Bosses on this list because they either have a unique evolution mechanic or are a "semi-regional."

In any case, more Raid Bosses for Level 1 Raids are needed. This is why Pokémon like Wigglytuff and Ivysaur appear on this list. Diversity in Raids will keep players interested for a longer period of time and keep too many rare, overpowered Pokémon from becoming too common.

Good luck in your Raid Battles, Trainers!