>>21907626If you check Prof. Oak's PC in his lab, there's an email from E4:
"Calling all
POKéMON trainers!
The elite trainers
of POKéMON LEAGUE
are ready to take
on all comers!
Bring your best
POKéMON and see
how you rate as a
trainer!
POKéMON LEAGUE HQ
INDIGO PLATEAU
PS: PROF.OAK,
please visit us!"
It sort of gets a vibe that the E4 is a recently opened competition or institution and that they're doing some "advertisement" for it, so probably there wasn't an actual Champion when Blue defeated Lance.
Also, there's is some leftover data in the game where Oak has the same team as Blue, but higher leveled, so probably he was planned to be the actual or former Champion early in the game, or simply a very strong trainer (hence why the mail, they wanted him to challenge them) or you had to defeat him to have granted access to Cerulean Cave.
