An adjective for an inconspicuous event that later lead to great things?
In the embryonic stage is an expression that may suggest the idea of something that is just at its earliest stages of development:
- in an early stage; rudimentary; undeveloped.
- of an organism prior to birth or hatching; "in the embryonic stage";
The Free Dictionary
From Istitutionalization of State Policy:
- Although the Internet in China is still in its embryonic stage, its development has ...
I'll suggest not a word but a phrase.
Seemingly insignificant
Seemingly unnotable
Seemingly implies that it had the appearance of X, but X was not necessarily the case.
“Late-blooming” wouldn’t describe the initial event (as you seek), but you could use it to describe the entire phenomenon (third full paragraph re "Family Guy" as a "late-blooming phenomenon" from USA Today).
adjective
of or characteristic of a late bloomer: ‘late-blooming brilliance.’
late in coming about or showing full development: ‘the country's late-blooming interest in soccer.’ – (Dictionary[dot]com)